The correct answer is (A).
The two possible carbon skeletons for four carbon atoms are: a straight chain C–C–C–C (butane) and a branched chain with three carbons in a row and one carbon branching off the middle carbon.
Source: Chapter 4, Section 4.2.2 (Chains, Branches and Rings)
The textbook explicitly states (Section 4.2.2, Figure 4.8) that butane (C₄H₁₀) has exactly two possible carbon skeletons: a straight chain (n-butane) and a branched chain where one carbon branches off the second carbon of a three-carbon chain (isobutane). Option A correctly shows these two skeletons. Options B, C, and D show incorrect or impossible branching arrangements for four carbons.