Optical isomerism occurs when a compound has a chiral center (asymmetric carbon).
Examining each compound:
n-propyl chloride (CH3CH2CH2Cl) has no chiral center,
iso-propyl chloride (CH3CHClCH3) has no chiral center,
sec-butyl chloride (CH3CH2CHClCH3) has one chiral center attached to four different groups, and
neo-pentyl chloride has no chiral center.
Therefore, sec-butyl chloride exhibits optical isomerism.
For C4H9Cl:
molar mass = 4(12) + 9(1) + 35.5 = 92.5 g/mol.
Carbon percentage = (48/92.5) × 100 = 51.89% ≈ 52%