In the following sequence of reactions the P is:

According to the question, In the first step of the reaction, when cyclo-pentyl-chloride reacts with magnesium in the presence of dry ether than cyclo-pentyl-magnesium-chloride (grignard reagent) is formed. 

In the second step of the reaction, when grignard reagent is react with a compound, which has active hydrogen than grignard reagent abstract hydrogen from active hydrogen compound and converted in to alkane. 

So cyclo-pentane is formed as the product(second step of the reaction) of this reaction.