Hello Everyone!
Let's tackle this question, one issue at a time, and narrow it down to the right answer quickly! To start, let's take a quick look at the original question and highlight any major differences between the options in orange:
Most efforts to combat such mosquito-borne diseases like malaria and dengue have focused either on the vaccination of humans or on exterminating mosquitoes with pesticides.
A. like malaria and dengue have focused either on the vaccination of humans or on exterminating
B. like malaria and dengue have focused either on vaccinating of humans or on the extermination of
C. as malaria and dengue have focused on either vaccinating humans or on exterminating
D. as malaria and dengue have focused on either vaccinating of humans or on extermination of
E. as malaria and dengue have focused on either vaccinating humans or exterminating
After a quick glance over the options, a few key differences jump out:
1. like vs. as (Idioms)
2. either on vs. on either (Parallelism & Idioms)
3. on exterminating / on the extermination of / on extermination of / exterminating (Parallelism)
Let's start with #1 on our list because it will knock out 2-3 options right away. This is an issue of idioms! In English, it's actually incorrect to introduce examples with the word "like." We use the word "like" in similes to show how to things are alike - not that they are examples of a larger category. To introduce examples, we use "as" or "such as." Here are some examples:
To save money, consider shopping at discount store like thrift shops and consignment stores. --> WRONG
To save money, consider shopping at discount stores, such as thrift shops and consignment stores. --> CORRECT
The air smelled like fresh dew. -->CORRECT
The air smelled as fresh dew. ---> WRONG
So let's see how our options hold up:
A. like malaria and dengue have focused either on the vaccination of humans or on exterminating
B. like malaria and dengue have focused either on vaccinating of humans or on the extermination of
C. as malaria and dengue have focused on either vaccinating humans or on exterminating
D. as malaria and dengue have focused on either vaccinating of humans or on extermination of
E. as malaria and dengue have focused on either vaccinating humans or exterminating
We can eliminate options A & B because they don't use the correct idiom structure to introduce examples. That was a quick way to eliminate 2 options, right?
Now that we've narrowed things down a bit, let's tackle #2 & #3 on our list. They actually have to do with the same grammar concept: parallelism!
We need to make sure that the two items that are being focused on are written using parallel structure:
C. as malaria and dengue have focused on either vaccinating humans or on exterminating
have focused on either X or on Y = NOT PARALLEL
D. as malaria and dengue have focused on either vaccinating of humans or on extermination of
have focused on either X or on Y = NOT PARALLEL
(This is also incorrect because the two items don't use the same wording: vaccinating & extermination)
E. as malaria and dengue have focused on either vaccinating humans or exterminating
have focused on either X or Y = PARALLEL
There you have it - option E is the correct choice! It's the only one that uses the proper idiom "as" to introduce examples, and it uses parallel structure when listing the two items being focused on.
Don't study for the GMAT. Train for it.