Hi Jenna,
The stimulus sets out a basic principle: no one should be appointed to a position if it involves duties that person should not perform. First of all, for simplicity, assume when I say "citizen" I mean "U.S. citizen," and when I say "non-citizen" I mean "foreign citizen."
It first applies that principle to cabinet secretaries. That job involves duties that only citizens should perform, so non-citizens (
i.e., foreign citizens) should not be cabinet secretaries.
Then it tells us that a cabinet
undersecretary sometimes has to fill-in as a cabinet secretary. We can assume that when that happens, they do all the same tasks, including the tasks that are reserved for citizens only.
Taking those two things together--to be an undersecretary, you also should be a citizen, because you might have to take over tasks that should be reserved for citizens only. Answer (B) just rephrases that by saying foreign citizens should not be appointed as cabinet undersecretaries.
The stimulus says "should" instead of something like "must," so it's not absolute, but the reasoning is largely the same as with any necessary/sufficient question (just make sure the right answer also says "should"). Here is what the diagram would look like if we substitute "must":
Secretary 
citizen-only duties citizen
Undersecretary citizen-only duties (when acting as secretary) citizen
citizen undersecretary (contrapositive)
I hope that's helpful!
