Structural isomers are compounds with the same molecular formula but different connectivity of atoms, and counting them requires systematically varying the carbon skeleton. For C_5H_12 the possible skeletons are the straight chain n-pentane, the singly branched 2-methylbutane (isopentane), and the doubly branched 2,2-dimethylpropane (neopentane). These three chain arrangements exhaust all unique connectivities, so there are exactly three structural isomers. The answer 2 is wrong because it omits the highly branched neopentane skeleton. The answer 4 overcounts by treating a renamed version of an existing skeleton as new, which violates the rule that identical connectivity is one isomer. The answer 5 grossly overcounts and cannot be realised with only five carbons. This classic isomer-counting exercise appears directly in NCERT for the pentanes and is a building block for later counting of positional and functional isomers in substituted compounds. A systematic method is to start from the longest chain, then progressively shorten it while adding branches in all non-equivalent positions, taking care never to recount a skeleton already drawn in a different orientation. The three pentane isomers also differ measurably in boiling point because branching reduces surface contact. Plausibility check: drawing each skeleton and confirming that no two share the same connectivity verifies that only three genuinely different structures exist for C_5H_12.