The speaker concludes that inbreeding is an underlying cause of the decline in honeybee populations. The only evidence given for this claim is that breeding practices have limited the genetic diversity of honeybees, and that there are several proposed immediate causes.
Although this may make sense to you on a quick, casual read of the argument, we need to be very critical of the speaker. Does limited genetic diversity necessarily weaken honeybees? We are not expected to know insect epidemiology for this exam, so we cannot make this assumption.
The author is leaving a gap in the argument. We are told that honeybees have limited genetic diversity, but we cannot necessarily link that to population declines. Whenever you see such a gap in a Necessary Assumption question, you should be on the look out for Supporter answer choices. We can tell exactly what the author thinks is trying to do with the information, but the elements are not linked together as well as they should be.
Answer Choice (E) links these together and tells us that limited genetic diversity can cause honeybees to be weakened int he face of conditions such as pesticides, bacteria, mites, etc...
If you are still not sure, apply the Assumption Negation Technique. First logically negate the answer choice at hand:
Lack of genetic diversity cannot make honeybees more vulnerable to adverse conditions.
If we were to learn that information, we could not accept the speaker's conclusion that breeding has done anything to cause populations to decline. For that reason, the speaker must assume that answer choice (E) is true.