Answer: Option C. -> $\text{Zn(s)}+\text{2HCl(aq)}\to {\text{ZnCl}}_2\text{(aq)}+{\text{H}}_2\text{(g)}$
:
C and D
In a decomposition reaction, the reactant breaks into two or more simple products. So, the reactions of calcium carbonate and potassium chlorate to form products are examples of decomposition reaction. 
The reaction between zinc and hydrochloric acid is a displacement reaction as zinc is displacing hydrogen.
The reaction between hydrogen and chlorine is a combination reaction as the reactants are combining to form hydrogen chloride.

Answer: Option C. -> ${\text{AgNO}}_3\text{(aq)}+\text{NaCl(aq)}\to \text{AgCl}\text{(s)}+{\text{NaNO}}_3\text{(aq)}$
:
C
The reaction in which exchange of ions takes place between the compounds of the reactants is called a double displacement reaction.
The reaction between silver nitrate and sodium chloride involves exchange of their ionic counter parts to form the product. Hence, it is a double displacement reaction.
All the other three reactions given are combination reactions. Combination reaction is a reaction in which two or more elements or compounds combine to form a new product.

Answer: Option A. -> decomposition
:
A
The conversion of calcium carbonate ${\text{(CaCO)}}_3$ to calcium oxide $\text{(CaO)}$ is an example of decomposition reaction. During the reaction, calcium carbonate decomposes on heating to form calcium oxide and carbon dioxide. The reaction takes place as follows:
${\text{CaCO}}_3\text{(s)}\stackrel{\text{Heat}}{⟶}\text{CaO (s)}+{\text{CO}}_2\text{(g)}$

Answer: Option C. -> $\text{Fe(s)}$ + ${\text{CuSO}}_{\text{4}}\text{(aq)}$$\to$ ${\text{FeSO}}_4\text{(aq)}$ + $\text{Cu(s)}$
:
C
The reaction in which a more reactive element displaces a less reactive element from its compound or salt solution is called a displacement reaction.
Among the given options only in the reaction between iron and copper sulphate, iron $\text{(Fe)}$ being more reactive displaces copper $\text{(Cu)}$ from its solution.

Answer: Option D. -> Iron nails dissolve completely.
:
D

When iron nails are added to an aqueous solution of copper sulphate, a displacement reaction occurs in which iron displaces copper from its solution which leads to fading of the blue colour of solution. After some time, the displaced copper settles down on the iron nails and reduces the surface available for reaction. So, the iron nails do not dissolve completely. The reaction involved is:
$\text{Fe}\text{(s)}+{\text{CuSO}}_4\text{(aq)}\to {\text{FeSO}}_4\text{(aq)}+\text{Cu (s)}$

Answer: Option D. -> $2\text{K}\text{(s)}+2{\text{H}}_2\text{O}\text{(l)}\to 2\text{KOH(aq)}+{\text{H}}_2\text{(g)}$
:
D
Potassium reacts with water to form potassium hydroxide along with hydrogen. The chemical equation for this reaction is given below:
$\text{K}\text{(s)}+{\text{H}}_2\text{O}\text{(l)}\to \text{KOH(aq)}+{\text{H}}_2\text{(g)}$
We can see that:
Reactant
Product
Elements   Symbol
  No. of    atoms
 Elements   Symbol
  No. of    atoms 
   K
  1
   K 
  1
   H
  2
   H
  3
   O
  1
   O
  1

Now in order to balance the equation the number of atoms on both reactants and the products side should be equal. By applying hit and trial method the balanced equation of the given reaction is:
$\text{2K}\text{(s)}+{\text{2H}}_2\text{O}\text{(l)}\to \text{2KOH (aq)}+{\text{H}}_2\text{(g)}$

Answer: Option A. -> mass is conserved in a chemical reaction.
:
A and B

Mass can neither be created nor destroyed in a chemical reaction, i.e. mass is conserved in a chemical reaction. This implies that the number of atoms before and after the reaction should be the same. So a chemical equation should be balanced.

Answer: Option A. -> ${\text{MnO}}_2$
:
A
An oxidising agent is a chemical compound that undegoes reduction in a redox reaction.  In the given reaction, manganese dioxide is behaving as an oxidising agent as it is getting reduced to manganese.

Answer: Option B. -> False
:
B
Oxidation involves addition of oxygen or removal of hydrogen. Reduction involves addition of hydrogen or removal of oxygen.
For example:
$\text{C(g)}+{\text{O}}_2\text{(g)}\to {\text{CO}}_2\text{(g)}$
In this reaction carbon ($\text{C}$) is getting oxidised, whereas carbon dioxide (${\text{CO}}_2$) is getting reduced.

Answer: Option B. -> $\text{MgO}+{\text{CO}}_2\to {\text{MgCO}}_3$
:
B and C
In a combination reaction two or more reactants combine to form a single product. The reactions involving formation of magnesium carbonate and calcium carbonate are examples of combination reaction as two reactants are combining to give a single product.
When Methane and Oxygen reacts, the bonds in the methane and oxygen come apart, the atoms rearrange and then re-bond to form water and carbon dioxide. It is an example of combustion reaction.
When silver chloride is exposed to sunlight, decomposition reaction takes place. AgCl is broken down into Ag and Cl.