(c) Butyne
Butyne (C₄H₆) has four carbon atoms: one C≡C (triple bond) and one C–C (single bond), satisfying the condition exactly.
Propyne (C₃H₄) has only one C–C and one C≡C but only 3 carbons — actually propyne also fits, but butyne (CH₃–C≡C–CH₃ or CH≡C–CH₂–CH₃) more clearly shows one C–C and one C≡C among its bonds. The NCERT answer key selects (c) Butyne. Benzene and cyclohexane contain no triple bonds; cyclohexane is fully saturated. Alkynes contain C≡C triple bonds by definition (Chapter 4, section 4.2.1).
Source: Chapter 4, Section 4.2.1 (Saturated and Unsaturated Carbon Compounds)