A functional group is an atom or a group of atoms responsible for the characteristic chemical properties of an organic compound.

a)

(i) A functional group is an atom or a group of atoms responsible for the characteristic chemical properties of an organic compound.

(ii)

• For compound B, the functional group is a carbon-carbon double bond ($\text{C}=\text{C}$). The homologous series is Alkene.
• For compound D, the functional group is a hydroxyl group ($\text{-OH}$). The homologous series is Alcohol.

Therefore:
B: Alkene
D: Alcohol