Which of the following set of compounds does not belong to same homologous series?

A homologous series is a series of compounds having the same functional group and general formula, where successive members differ by a $-CH_2-$ unit (14 u in molecular mass).

(A) $CH_4$ (methane, $n=1$) and $C_4H_{10}$ (butane, $n=4$): both fit the alkane general formula $C_nH_{2n+2}$ — same series.

(B) $C_2H_6$ (ethane, $n=2$) and $C_3H_8$ (propane, $n=3$): both fit $C_nH_{2n+2}$ — same series, differing by one $-CH_2-$ unit.

(C) $C_3H_8$ (propane, $n=3$) and $C_5H_{12}$ (pentane, $n=5$): both fit $C_nH_{2n+2}$ — same series (alkanes).

(D) $C_4H_8$ fits the alkene general formula $C_nH_{2n}$ (it is butene, an unsaturated hydrocarbon with one double bond), whereas $C_5H_{12}$ fits the alkane general formula $C_nH_{2n+2}$ (pentane, a saturated hydrocarbon). Since these two compounds belong to different general formulae (and hence different functional-group classes — alkene vs alkane), they do not belong to the same homologous series. This is the correct answer.

As the NCERT chapter states: 'such a series of compounds in which the same functional group substitutes for hydrogen in a carbon chain is called a homologous series,' and successive members of a homologous series 'differ by a $-CH_2-$ unit.' Options A, B and C all satisfy this rule for alkanes; option D violates it because the two compounds belong to different classes (alkene and alkane).