Thanks for the question! Let me see if I can help.
I believe in your comment you are referencing the Assumption Negation Technique, which is absolutely a tool you can and should be using with Assumption problems like this one. Let's use that with both answer choice (E) and the credited answer choice (B) and see if we can't figure out what this issue is.
Answer choice (E) reads: Human skin never gives off any gaseous substances that repel mosquitoes.
You note in your comment that the negation would be Human skin always gives off gaseous substances that repel mosquitoes.
Careful here! Recall that we are looking for the logical opposite of the answer choice - and the logical opposite of "never" is "sometimes" (not "always"). If you are a PowerScore student, make sure you review those logical opposites in the Bible or the Coursebooks...it doesn't end up mattering here, but just a note
Anyway, the negation of answer choice (E) ends up being Human skin sometimes gives off gaseous substances that repel mosquitoes.
Okay, so if this were true, would it weaken the author's conclusion that some other gaseous substance given off by human skin also attracts mosquitoes
? Not particularly...in fact, if human skin sometimes gives off gaseous substances that repel mosquitoes, it may even slightly strengthen the idea that human skin is giving off other gaseous substance(s) that attracts mosquitoes, since (in this scenario) the mosquitoes are still choosing to bite the human arm, perhaps leading to the conclusion that the substances attracting the mosquitoes are overcoming whatever negative ones that repel the mosquitoes.
Answer choice (B) reads: Mosquitoes are not attracted to humans by body heat.
This one is fairly easy to take the logical opposite of, it would read Mosquitoes areattracted to humans by body heat.
If this were true, then, would it weaken the argument? It would: if mosquitoes are attracted by the body heat of humans, it weakens the idea that there must be a gaseous substance that is attracting them (it could just be the heat). Therefore, answer choice (B) is credited.
Hope that helps!