The Joint Chiefs of Staff are in charge of A. the day-to-day running of the military B. advising Congress on military matters C. the White...

None of these answers is really a good fit with what the Joint Chiefs of Staff (JCS) actually do.  Let us look at what the JCS’s website says about their mission.  According to the site (in the link below),

The Chairman of the Joint Chiefs of Staff is the principal military adviser to the President, Secretary of Defense, and the National Security Council (NSC), however, all JCS members are by law military advisers, and they may respond to a request or voluntarily submit, through the Chairman, advice or opinions to the President, the Secretary of Defense, or NSC.

You will notice that this description of their mission does not really match any of the options in this question.  I would say that Option B is the best answer, except that it mentions Congress rather than the President and other parts of the executive branch.

Option A is clearly not correct because the JCS are not part of the command structure.  They no longer have a role in ordering overall military operations, let alone day-to-day operations.  Option C is not right because White House security is a very narrow duty compared to what the JCS actually does.  Option D is wrong because the JCS have nothing to do with espionage. Option E is wrong because the JCS is an advisory body, not a coordinating body.

Therefore, if I had to pick one of these, I would pick Option B because it is closest to being correct.