According to this sentence, there are two ways in which linguists can learn about how children learn a language:

(1) From the mistakes that children make
(2) From the correct forms used by children

Linguists learn more from the mistakes made by children than from the correct forms.

Hope that helps!
BTW, I think the OA should be C. Is it so?

The mistake children make in learning to speak tell linguists more
about how they learn language than the correct forms they use

a. how they learn language than
b. how one learns language than
c. how children learn language than do
d. learning language than
e. their language learning than do

It appears there is a typo: "The mistake" should be a plural "The mistakes"
to match the corresponding verb "tell"
"how they (children) learn than the correct form they (children) use.
At first, "they" seems to be ambiguous in that the pronoun could refer to either
children or linguists. however, linguists are not learning the language.
I'd rather opt for A.

Dude, only [C] can be right answer. "they" used in the original sentence brings ambiguity and has to be replaced with "children" for clarity.