which compound reacts with bromine most easily ? alkene or

It is the fact that naturally accurance of bromine in liquid phase but here we have given Gaseous phase. e.g., Br₂(l) + ∆ ----> Br₂(g) 
Here you see heat requires to change the phase of bromine from liquid to gas. it means reaction take place higher temperature so, machenism with Br₂ should be free radical substitution , which can take place in propene and pentane.

Also you know, reaction of free radical depends upon stability of free radical.
for propene : CH₃-CH=CH₂ ⇔ CH₂-CH•=CH₃ { it is stablized by conjugation }
For pentene : CH₃ - CH₂ - CH₂ - CH₃ ⇔CH₃- CH - CH₂•-CH₃ 

Hence, propenyl free radical is more stable . 
So, answer is C₃H₆

 

Hope it helps

 

Regards

Arun (askIITians forum expert)