Bromine (Br₂) decolourization is a test for unsaturation. The number of moles of Br₂ decolourized depends on the number of double bonds or triple bonds present. A double bond reacts with 1 mole of Br_{2, while a triple bond reacts with 2 moles of Br2.}

Let's analyze the moles of Br₂ consumed per mole of compound:

• Option 1 (Cyclohexene): 1 double bond → 1 mole Br₂
• Option 2 (Acetylene, HC≡CH): 1 triple bond → 2 moles Br₂
• Option 3 (Propene): 1 double bond → 1 mole Br₂
• Option 4 (1,3-Butadiene): 2 double bonds → 2 moles Br₂

Both acetylene and 1,3-butadiene decolourize 2 moles of Br₂ per mole of compound.

Final Answer: HC≡CH and