Answer: Option C. -> prop
:
C
The word roots are selected based on the number of carbon atoms in the parent chain of the hydrocarbon. Given below are few word roots corresponding to the number of carbon atoms in the parent chain of the hydrocarbon.
$$\begin{array}{cc}\text{Number of carbon atoms}& \text{Word root}\\ \text{One}& \text{Meth}\\ \text{Two}& \text{Eth}\\ \text{Three}& \text{Prop}\\ \text{Four}& \text{But}\end{array}$$

Answer: Option B. -> allotropy
:
B

Allotropy is the property of some chemical elements to exist in two or more different forms in the same physical state.
Allotropes are the different structural modifications of an element, i.e.,  atoms are bonded together in different manner or arrangement.

Answer: Option B. -> It's an alkene
:
B and C
The given compound is an unsaturated hydrocarbon because it contains a $\text{C=C}$ bond. It is an alkene having the general formula ${\text{C}}_{\text{n}}{\text{H}}_{\text{2n}}$. The given compound is numbered as shown below.
$\underset{6}{{\text{CH}}_3}-\underset{5}{{\text{CH}}_2}-\underset{4}{{\text{CH}}_2}-\underset{3}{\text{CH}}=\underset{2}{\text{CH}}-\underset{1}{{\text{CH}}_3}$
$\bullet$ The compound has six carbon atoms in it and hence, the word root is 'hex'.
$\bullet$ It has a double bond between the second and the third carbon atoms and hence, the suffix will be '2-ene'.
Therefore, the IUPAC name of the given compound is hex-2-ene.

Answer: Option B. -> Successive members of a homologous series differ by a ${\text{CH}}_3$ group.
:
B
The characteristics of homologous series are:
(1) The members can be represented by the same general formula. For example, ${\text{C}}_n{\text{H}}_{2n}$ for alkenes.

(2) Successive member differs by a ${\text{CH}}_2$ group and not by a ${\text{CH}}_3$ group.

(3) Members show similarity in chemical properties.

(4) There is a regular gradation in physical properties. For example, in alkanes as 'n' value increases, their boiling point increases. 

Answer: Option C. -> aldehyde
:
C
The given compound has the formula ${\text{CH}}_3{\text{CH}}_2\text{CHO}$. As it has a -CHO group at terminal position, its IUPAC nomenclature is propanal. The functional group -CHO represents an aldehyde.

Answer: Option C. -> Methanol, Ethanol, Propanol, Butanol
:
C
A homologous series is a series of carbon compounds that have different number of carbon atoms but contain the same functional group and the consecutive members differ by a ${\text{CH}}_2$ group.
Methanol, ethanol, propanol and butanol are all alcohols and hence, belong to the same homologous series. They differ from each other by one ${\text{CH}}_2$ group.
In all the other options, even though they have same functional groups, the type of C-C bonds are different.