Hello Everyone!
Let's tackle this question, one issue at a time, and narrow it down to the right option quickly! To begin, here is the original sentence with any major differences between the options highlighted in orange:
Nobody knows exactly how many languages there are in the world, partly because of the difficulty of distinguishing between a language and the sub-languages or dialects within it, but those who have tried to count typically have found about five thousand.
(A) and the sub-languages or dialects within it, but those who have tried to count typically have found
(B) and the sub-languages or dialects within them, with those who have tried counting typically finding
(C) and the sub-languages or dialects within it, but those who have tried counting it typically find
(D) or the sub-languages or dialects within them, but those who tried to count them typically found
(E) or the sub-languages or dialects within them, with those who have tried to count typically finding
After a quick glance over the options, there are a few major differences we can focus on:
1. it vs. them (pronoun agreement)
2. to count / counting (verbs or meaning)
3. have found / finding / find / found (verb tense)
Let's start with #1 on our list: it vs. them. No matter which one we choose, it will eliminate 2-3 options right away. To figure out which one we need to use, let's ask ourselves what the pronoun it/them is referring to. So...what are sub-languages and dialects considered "within?"
Nobody knows exactly how many languages there are in the world, partly because of the difficulty of distinguishing between a language and the sub-languages or dialects within it, but those who have tried to count typically have found about five thousand.
There you go! The pronoun is referring back to the singular word "language!" Now, let's rule out any options that use a plural pronoun instead:
(A) and the sub-languages or dialects within it, but those who have tried to count typically have found
(B) and the sub-languages or dialects within them, with those who have tried counting typically finding
(C) and the sub-languages or dialects within it, but those who have tried counting it typically find
(D) or the sub-languages or dialects within them, but those who tried to count them typically found
(E) or the sub-languages or dialects within them, with those who have tried to count typically finding
We can rule out options B, D, & E because they use the plural pronoun "them" to refer to a singular noun "language." Pretty easy, right? We just eliminated 3 options really quickly!
Now that we've got this narrowed down to only 2 options, let's tackle #2 & #3 on our list. They both have to do with finding the right verb tenses and creating a clear, concise meaning, so let's read them over and see which one does this better:
(A) and the sub-languages or dialects within it, but those who have tried to count typically have found
This is CORRECT! It doesn't have any problems with pronoun agreement, the verbs make logical sense, and the meaning is clear!
(C) and the sub-languages or dialects within it, but those who have tried counting it typically find
This is INCORRECT because there is an agreement problem with the pronoun "it" that people are trying to count. The pronoun "it" is singular, but it's referring to the plural phrase "sub-languages or dialects."
There you have it - option A was the correct choice all along!
Don't study for the GMAT. Train for it.