By building military bases around the world, imperialist nations mainly aimed to a: resupply naval vessels. b: initiate wars in several places at once.

B) initiate wars in several places at once . . . assuming they have sufficient resources to supply each base, it is good to have deployment available at your fingertips when needed

If those are really the only two choices, under normal circumstances, it would be A. The reason is that bases make it easier to resupply vessels and also provide a military presence/protection. While in B, as a former military strategist studier, it would be a foolish nonstrategic reason. As you can see in past history, spreading yourself thin fighting against many countries in various areas prove unwise. Spreading yourself thin creates the possiblity of being overrun/undersupplied which will result in military loses. An example would be the U.S collision in the Persian Gulf war. The U.S won with sheer numbers along with better supplied and better trained/equipped who were also backed by its allies that Iraq never had a chance.