(i) Since the first member of the alkyne series has 2 carbon atoms in it, therefore, third member of the alkyne series will have 4 carbon atoms in its molecule.

So, if we substitute n = 4 in the general formula C_nH_{2n – 2} then the molecular formula of the third member of the alkyne series will be C₄H_{2 × 4 – 2} or C₄H₆.

(ii) Since the first member of the alkyne series has 2 carbon atoms in it, therefore, the fifth member of the alkyne series will have 6 carbon atoms in it.

So, if we substitute n = 6 in the general formula C_nH_{2n – 2} then the molecular formula of the fifth member of the alkyne series will be C₆H_{2 × 6 – 2} or C₆H₁₀.

All these compounds are hydrocarbons, so all that we have to do is to find out which of them are alkanes, alkenes and alkynes.

(i) C₃H₈ corresponds to the general formula for alkanes C_nH_{2n + 2} (with n = 3), therefore, C₃H₈ is an alkane.

(ii) C₃H₆ and C₄H₈ correspond to the general formula for alkenes C_nH_2n (with n = 3 and n = 4, respectively), therefore, C₃H₆ and C ₄H₈ are both alkenes.

(iii) C₄H₆ corresponds to the general formula for alkynes C_nH_{2n – 2} (with n = 4), therefore, C₄H₆ is an alkyne.

From the above discussion it is clear that C₃H₆ and C₄H₈ belong to the same homologous series of alkenes.