GMATist1I'm not sure which of the previously posted reasons you have in mind, but let me discuss "may actually increase" in some detail.
First, why "may"? Well, first notice that 4 out of 5 answer choices say "may," so it's no accident. We can figure that the author probably wants to say "may," so unless it creates a grievous error of meaning, we ought to keep it. But what is its function? After all, the studies are only "suggesting" in the first place, right? It's important to note that it's not there to add uncertainty to the result--"suggest" already does that--but to indicate that outcomes may vary. For instance, I know for a fact that exceeding the speed limit MAY lead to a speeding ticket. I'm not in any doubt about how speeding tickets work; I just know that not everyone gets a ticket. If we choose B, we are saying that any time anyone consumes an excess of vitamin E, their risk of disease increases. This may not be true. Health is complicated, and the outcome may depend on hundreds of other factors.
What about "actually"? That's there to highlight the contrast between the claims or expectations of those who "swear by it as a dietary supplement" and the result presented in the sentence. In fact, we might use "actually" even if that last part of the sentence weren't there, since it's common knowledge that vitamins are supposed to be good for you. So basically, the sentence is saying "People think taking a lot of vitamin E is good for you, but ACTUALLY it can harm you if you take more than needed."
Then there's "increase." That's the one part that's simple grammar. When we say that something may happen, we use the "bare infinitive" form of the verb: the infinitive minus "to." In fact, I did it just know when I said "may happen." We wouldn't say "may happens." This is common with helping verbs.
A few examples:
This shot
may pinch a little.
Leslie
can see the lake from her house.
Vijay
will go to the bookstore later.