reply2spg has already explained it better.
B is the closet answer, but you are right. It's little bit far from inference. However, if you consider the below inferences, it would be clear that B is the closet one and in a way validates the choice.
Inference 1 - We can infer taht France had access to Cumberland graphite before war ( when?? we don't know, but we know this from the last sentence)
Inference 2 - France was using solid graphite for pencil ( France may be using other graphite, which we don't know. But from first sentence, we know that solid graphite was mined from Cumberland and used in the early pencil)
Now, since we know that France had access to cumberland and was using graphite before, it must find the alternate source. B fits the bill since the argument mentioned that Frace had to fund research to find the alternate source. Hope, this helps.
