You're on the right track. Let's focus on the following two conditional statements in the stimulus:
1. Often the pollinating species (i.e. bees) can pollinate only a single species of plant. In other words, some bees depend on a single species of plant:
Bee species (x)
Plant species (x)
2. Some plant species have evolved flowers that only a single species of bees can pollinate. In other words, some plants depend on a single species of bees:
Plant species (y)
Bee species (y)
The last sentence tells us what happens when bee species (y) gets destroyed: plant species (y) can no longer reproduce. This is the contrapositive of the second statement above. The same thing will happen if plant species (x) is destroyed: bee species (x) will not survive either. Since (x) simply stands for "some" plant species, answer choice (E) can easily be proven by the contrapositive of the first conditional statement above.
Answer choice (B) is a Mistaken Reversal: just because the sole pollinator of some plant species (bee species [y]), is thriving doesn't mean that plant species (y) will thrive as well. The plants can be destroyed by some other means (pesticides, infections, etc.) even if their only pollinator is doing OK. Makes sense?
Thanks!