This question is asking for a necessary assumption. Let's ID the core:

The conclusion is that Lieniz and Newton each independently discovered calculus.

Why? What's the premise? It takes a bit of digestion, but the premises boil down to this:

Liebniz published his version of calculus before Newton, and didn't receive any real help (regardless of some people who disagree). Also, Newton had been using those ideas for at least a decade before Lieniz published his. All of that boils down to the idea that they didn't help each other.

There are some serious gaps here! For one, just because Liebniz published his ideas in year X doesn't mean he didn't tell Newton about it beforehand. Similarly, we never learn that either character actually discovered calculus. Perhaps they didn't help each other, but who is to say that the fact these two guys didn't help each other means they each discovered calculus? Maybe their wives did and they stole the work (independently of each other, of course

).

(E) hinges on this assumption - we have to assume that neither of them learned crucial bits of calculus from someone else.

(A) is about who Liebniz told before publishing it. It's a problem of Liebniz told Newton, but it might be OK if he told someone else.

(B) This is quite tempting! It sure seems that the negation of this - a third person independently discovered calculus - would destroy the argument. But it doesn't! So what if a third person independently discovered it? As long as neither Newton or Liebniz used that discovery to "discover" calculus, it's not a problem. Apparently this argument accepts the idea that multiple people can independently discover an idea.

(C) is irrelevant - we don't care about what Newton believed about Liebniz.

(D) is also tempting - it sounds like it's saying "neither learned calculus from the other" but it doesn't! It says that neither knew that the other had discovered a version of calculus. Knowing that someone had developed something doesn't mean you know what the thing is.