13C isotopic ratio reveals number of carbon atoms Steps:

• Ratio M:M+1 = 13:1 gives (M+1)/M = 1/13 ≈ 0.077 (7.7%).
• Each carbon contributes 1.1% to M+1 abundance, so n_C ≈ 7.7 / 1.1 = 7 carbons.
• Option A (butyl butanoate, C8H16O2) has 8C; B (hexan-3-one, C6H12O) has 6C; C (2,3-dimethylpentan-1-ol, C7H16O) and D (3-methylhexane, C7H16) have 7C.
• Alcohols like C fragment easily, yielding weak M/M+1 peaks; alkanes like D show prominent peaks for reliable ratio. Why D is correct:
• 3-Methylhexane has 7 carbons and, as an alkane, displays stable molecular ion per mass spectrometry principles. Why the others are wrong:
• A: 8 carbons predict M:M+1 ≈ 11:1.
• B: 6 carbons predict M:M+1 ≈ 15:1.
• C: 7 carbons but alcohol's weak M peak prevents accurate ratio.

Final answer: D