Which one of the following hydrocarbons is different from the others?

To identify the odd hydrocarbon, we analyze their molecular formulas using the general formulas of homologous series:

1. Alkanes (Saturated Hydrocarbons): General formula is $C_nH_{2n+2}$.

• For $n = 2$: $C_2H_{2(2)+2} = C_2H_6$ (Ethane) -> Alkane
• For $n = 4$: $C_4H_{2(4)+2} = C_4H_{10}$ (Butane) -> Alkane
• For $n = 5$: $C_5H_{2(5)+2} = C_5H_{12}$ (Pentane) -> Alkane

1. Alkenes (Unsaturated Hydrocarbons): General formula is $C_nH_{2n}$.

• For $n = 7$: $C_7H_{2(7)} = C_7H_{14}$ (Heptene) -> Alkene

Since $C_7H_{14}$ is an alkene containing a double bond, it is unsaturated and chemically more reactive than the other three saturated alkanes. Thus, Option B is the correct choice.