Answer: Option D. -> redox reaction
:
D

• In the given reaction, oxidation and reduction is occuring simultaneously as carbon dioxide is reduced to carbon monoxide by losing oxygen and hydrogen is oxidised to water by gaining oxygen.


• The reaction in which oxidation and reduction occurs simultaneously is called a redox reaction.

Answer: Option B. -> Combination reaction
:
B
In combination reaction, two or more elements or compounds react to form a single product. The given reaction is a combination reaction as carbon and oxygen are combining to form carbon dioxide.

Answer: Option C. -> $2\text{Na}\text{(s)}+2{\text{H}}_2\text{O(l)}\to 2\text{NaOH(aq)}+{\text{H}}_2\text{(g)}$
:
C
The chemical reaction in which a more reactive element displaces a less reactive element from its compound is called displacement reaction.
Among the given options, the reaction between sodium and water to form sodium hydroxide and hydrogen gas is an example of a displacement reaction. This is because sodium, which is more reactive than hydrogen, displaces it from water.

Answer: Option B. -> Rusting of iron
:
B
Rusting of iron is a chemical reaction. When iron rusts, it reacts with oxygen present in water or moisture in the air to create a new compound called iron (III) oxide.
$Fe\left(s\right)+O_2\left(g\right)\to F{e}_2{O}_3.x{H}_{2}O\left(s\right)$
$F{e}_2{O}_3.x{H}_{2}O$ is commonly known as rust and has properties entirely different from that of iron or oxygen. 
Other options involve either change in physical state or size.

Answer: Option C. -> Hydrogen gas and iron (III) chloride are formed
:
C
In an aqueous solution, the hydrochloric acid dissociates to render hydronium ions and chloride ions. The released chloride ions oxidize the iron atoms to form iron (III) chloride as ${\text{Fe}}^{3+}$ ions are more stable than ${\text{Fe}}^{2+}$ ions. The hydrogen ions are reduced to hydrogen gas. Here, the reduction is observed in terms of the gain of electrons and the oxidation is observed in terms of the loss of electrons. Hence, hydrogen gas and iron (III) chloride are produced.

Answer: Option C. -> endothermic
:
C

When potassium permanganate is heated it produces potassium manganate, manganese dioxide, and oxygen gas. This means in the reaction heat is absorbed so it is an example of an endothermic reaction.

Exothermic reactions are those in which the heat is released.

Electrolysis is a process in which a substance undergoes decomposition by absorbing electrical energy.

Chemical reactions in which electrical energy is consumed are called electrochemical reactions.

Answer: Option D. -> (a) and (b) both
:
D
Often, a precipitate forms in a double displacement reaction. In this reaction-
$AgN{O}_3+NaCl\to AgCl+NaN{O}_3$
both $AgN{O}_3$ and $NaCl$ exchange their ions to form new compounds, $AgCl$ (ppt) and $NaN{O}_3$. Hence, it shows both double displacement and precipitation.

Answer: Option D. -> ${\text{NH}}_3\text{(s)}+\text{HCl (aq)}\leftrightharpoons {\text{NH}}_4\text{Cl (s)}$
:
D

While writing a chemical equation letter are used in brackets or subscripts next to the chemical formulae to specify the physical state of the reactants and the products. 

- s is for solids

- l is for liquids

- g for gas

- aq is aqueous solution

The reaction in which the physical state of the reactants or the product is not correctly represented is:

${\text{NH}}_3\text{(s)}+\text{HCl (aq)}\leftrightharpoons {\text{NH}}_4\text{Cl (s)}$

This is because ammonia $\text{(}{NH}_3)$ is a gas.

Answer: Option D. -> Combustion of methane gas
:
B, C, and D

Dissolution of sodium hydroxide in water, dilution of sulphuric acid and combustion of methane gas are exothermic reactions in which a large amount of heat is released.

Evaporation of water is endothermic reaction in which heat is absorbed.

Answer: Option D. -> redox reaction
:
D

• In the given reaction, oxidation and reduction is occuring simultaneously as carbon dioxide is reduced to carbon monoxide by losing oxygen and hydrogen is oxidised to water by gaining oxygen.


• The reaction in which oxidation and reduction occurs simultaneously is called a redox reaction.