Cyclopropane instability to addition reactions In a question I saw comparing the reactivity (to addition reactions) of cyclopropane, cyclobutane and cyclopentane, it was said that cyclopropane was most prone to addition reactions.

Ring strain does make it prone to reactions, but how can addition reactions make it any more stable?

In that context, how can you compare the three?
You're absolutely right: cyclopropane's maximum ring strain makes it highly prone to addition reactions. Keep the following definition in mind:


  An addition reaction, in organic chemistry, is in its simplest terms an organic reaction where two or more molecules combine to form a larger one (the adduct).


In your case:


  


Thus, it breaks down the ring into a straight chain, which makes the low stability of the ring a clear contributing factor to the reaction. 

References: Cyclopropane, wikipedia||
Addition reactions, wikipedia