Officail Approach

It's very easy to misread the conclusion of this passage as "killing all mosquitoes would eradicate malaria." After all, that makes the most sense because who wants to be bitten by any kind of mosquito? However, the conclusion is actually more specific than that:

"If all the mosquitoes carrying malaria are exterminated"

In other words, only certain mosquitoes are to be killed: those carrying the malaria parasite.

Thus, this may already trigger in your mind a key question about the conclusion: what about the other mosquitoes that will still be alive?

Evaluating the Answer Choices

(A) is the correct answer. The epidemiologist's plan is to kill only those mosquitoes that carry the malaria virus. However, all the non-infected mosquitoes will still be flying around and biting people. If a person is infected with malaria, then that means that they have the malaria parasite in their blood. So when a non-infected mosquito bites that person, that mosquito will now carry the malaria parasite and will be able to freely spread the virus to new humans it bites. In other words, answer choice (A) exposes a major flaw in the plan: killing only certain mosquitoes will be meaningless since the remaining mosquitoes are still able to transfer the disease from human to human. The cycle continues and nothing has changed.

(B) may be both confusing and tempting since it looks a lot like answer choice (A). However, (B) can ultimately be paraphrased as "there's only one way to pass malaria from one human to another: mosquito bites." In other words, this answer choice is just ruling out other possible transfer mechanisms, such as a non-infected person touching the blood of a person infected with malaria. So, this answer choice is really just giving us an interesting fact: only mosquitoes can spread malaria between humans. However, this does not attack the epidemiologist's plan specifically like (A) does. (A) emphasizes that there's still a way for an initially non-infected mosquito to spread malaria, which highlights the main flaw in the argument (not killing all the mosquitoes instead of just the ones currently with malaria).

Most Commonly Selected Wrong Answer:

(C) is tempting for two reasons. First, it says that malaria is "endemic" in many parts of the world, meaning that it's commonly found all over the planet. This makes it sound like it would be very hard to eliminate. However, just because eliminating malaria would be difficult doesn't mean it would be impossible. Secondly, it says that "many health workers" don't believe in the conclusion of the argument, which seems to be saying the conclusion isn't sound. However, this is just the opinion of a group of people. This answer choice therefore doesn't actually give us any kind of fact or evidence that the epidemiologist's plan won't work.

(D) is off topic. The argument is concerned with malaria and mosquitoes, but this answer choice is talking about a completely different disease (sickle-cell anemia). Furthermore, people who are "immune to mosquitoes" wouldn't be a concern to the epidemiologist since those people wouldn't be able to contract malaria from the mosquitoes anyways.

(E) is also off topic. The argument is specifically about eliminating malaria and not other viruses. This also doesn't explain why the epidemiologist's plan would not work.

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