Which of the following is NOT one of the first four members of the alkane series?

The alkane series, also known as saturated hydrocarbons, consists of compounds that contain only carbon and hydrogen atoms bonded together by single bonds and follow the general formula CnH2n+2. The first four members of the alkane series are methane (C1), ethane (C2), propane (C3), and butane (C4).

In this context, pentane (C5) is actually the fifth member of the series. This means that it does not belong among the first four alkanes. The other three choices—propane, butane, and methane—are indeed the first four alkanes. Propane is the third member, butane is the fourth, and methane is the first. Thus, choosing pentane correctly identifies the compound that is not one of the first four members of the alkane series.