There are two different situations in which semicolons are used - that makes this question particularly tricky!

One way that a semicolon is used (by far the most common way) is to separate two independent clauses. For example:
The GMAT can be frustratingly difficult to master; it requires knowledge of many arcane grammar rules.

In this case, we can think of it almost like a period. If we swapped a period in for that semicolon, we'd have two complete sentences on either side.

The other situation that requires a semicolon is actually unrelated to clauses. Consider the following sentences:
She has traveled to London, England, Tokyo, Japan, and Mumbai, India.
She has traveled to London, England; Tokyo, Japan; and Mumbai, India.

In the first sentence, it looks like we have a list of 6 places... but with the "and" in the wrong place. In the second sentence, though, it's clear that we're talking about 3 cities. If there is a list in which the list elements already have commas in them, we have to use semicolons to separate our list items. You can think of these semicolons as "super-commas"!

In this sentence, the GMAT is trying to trick us into thinking that we're looking at the first semicolon situation. There is an independent clause before the semicolon, so we think that we want another independent clause after it: something that says "they pollinate and produce" rather than "pollinating," which would be a modifier. When we look at D and E, however, we see that there's a second semicolon, and what comes after it is a subordinate clause. So, D and E are wrong.

Looking back at A, B and C, it's clear that we're looking at the second situation: the "super-comma" list. Any time there's a list, we know that we have to make the list parallel. So what are our list elements?
aiding... pollinating... and helping...

In A and C, we have "help" and "they help" respectively, so we don't have a parallel list. B is the only one that has "aiding," "pollinating," and "helping."

Answer: B