Saturated hydrocarbon with unique structure Steps:

• Not decolourising bromine in the dark means no C=C or C≡C bonds, so Q is an alkane (general formula CnH2n+2).
• "Has no isomers" means the formula allows only one constitutional (structural) isomer.
• Options A (CH4), B (C2H6), and C (C3H8) each have one possible structure; D (C4H10) has two (n-butane and 2-methylpropane).
• Thus, A, B, or C fit both conditions exactly.

Why B is correct:

• C2H6 (ethane) is an alkane with formula allowing only one structure (H3C-CH3), no branching possible, and undergoes substitution (not addition) with Br2, requiring light/heat.

Why the others are wrong:

• A: CH4 also fits both conditions (single structure, saturated).
• C: C3H8 also fits both conditions (single straight-chain structure, saturated).
• D: C4H10 has two isomers, violating the "no isomers" condition.

Not enough information to select only one option.

Final answer: Not enough information.