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.1) Distinguishing between a language and sub-languages or dialects within it -> it is correct as the sub-languages or dialects are under a single language.
A. and the sub-languages or dialects within it, but those who have tried to count typically have found
CorrectB. and the sub-languages or dialects within
them, with those who have tried counting typically
finding1) Point number 1
2) Finding is a gerund and not a verb. We require a verb here
C. and the sub-languages or dialects within it, but those who have tried counting
it typically
find1) "Count it". It refers to a single language. How a person count a single language.
2) Find -> Simple present and it provides a meaning as if the number of count will always be true. So it I count the number of languages 10 years from now, we will still get 5000. Which is not the meaning of the sentence.
D.
or the sub-languages or dialects within
them, but those who tried to count them typically found
1) them -> Point # 1 above
2) Between takes "And" and not "or"
E.
or the sub-languages or dialects within
them, with those who have tried to count typically
finding 1) them -> Point # 1 above
2) Between takes "And" and not "or
3) Finding is a gerund and not a verb. We require a verb here