Compound Q is the alkane C4H10

Steps:

• Three C-C single bonds connect four carbon atoms in the molecule.
• Ten C-H bonds attach to these carbons, giving ten hydrogen atoms.
• The formula C4H10 matches a saturated hydrocarbon (alkane) with general formula CnH2n+2 for n=4.
• Alkanes undergo free radical reactions with halogens.

Why D is correct:

• Alkanes like C4H10 react with Cl2 under UV light via free radical substitution, replacing H with Cl (e.g., CH3CH2CH2CH3 + Cl2 → CH3CH2CH2CH2Cl + HCl).

Why the others are wrong:

• A: Alkanes lack double bonds needed for addition polymerisation.
• B: Saturated compounds do not decolourise bromine water, which tests for unsaturation.
• C: C4H10 has only two isomers (n-butane and 2-methylpropane).

Final answer: D