2) C

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.