Reaction involving opening of epoxides
As you are dealing with Grignard reagents, the carbocation argument is inadequate since the reaction is in basic media (carbocations are brønsted acids, so they would be easily deprotonated to produce an alkene). In fact, this mechanism is concerted (everything happens simultaneously, not in 2 distinct steps such as in the formation of a carbocation followed by a nucleophilic attack).



The Grignard reagent could attack either the methylene $\ce{(CH_2)}$ carbon or the methylidyne $\ce{(CH(CH_3))}$ carbon. The methylene carbon is attacked preferentially because its substituents are smaller than the methylidyne ones (2 hydrogens against one hydrogen and a methyl group). So steric hindrance is the key factor for predicting the product correctly in this question, not carbocation stability. 

ps.: I drew a specific enantiomer just for visualization, but in this case, the absolute configuration doesn't matter.