Compound X is an unsaturated hydrocarbon (alkene)

Steps:

• Analyze the dot-and-cross diagram to identify atoms and bonds: three carbons, six hydrogens, with a C=C double bond.
• Determine molecular formula: C3H6, indicating unsaturation (general formula CnH2n for alkenes).
• Confirm structure: CH2=CH-CH3, propene, with pi bond in double bond.
• Predict reactivity: C=C bond allows addition reactions, including with bromine.

Why C is correct:

• Unsaturated alkenes add bromine across the C=C bond, forming a colourless dibromo compound and decolourising orange bromine water (addition reaction).

Why the others are wrong:

• A: Presence of C=C double bond means it is unsaturated, not a saturated hydrocarbon (CnH2n+2).
• B: Butane is C4H10; C3H6 has different molecular formula, so not an isomer.
• D: Propane is C3H8 (saturated alkane); this is C3H6 (unsaturated alkene, propene).

Final answer: C