Only alkenes are the organic compounds which turn orange-brown bromine water to colourless.
Alkenes have double bonds and have a formula in which there are double the number of other atoms as there are carbon atoms.
e.g. C₂H₄, C₃H₆, C₄H₈
1 – C₂H₄ is an alkene so it will decolourise bromine water
2 – propene is an alkene but polypropene is not as it does not have a double bond.
3 – CH₂ C(CH₃)₂ can be simplified to C₄H₈. This is an alkene and hence it will decolourise bromine water.
4 – CH₃CH₂I is a halogenoalkane and so, it will not decolourise bromine water.
Only 1 and 3 are alkenes. Hence C is the answer.