gmat_perfect wrote:Although the turtle has been toothless for more than 150 million years, in some contemporary turtle species the moderately sharp and jagged edges of their horny jaws function for teeth.

(A) their horny jaws function for teeth
(B) its horny jaws function for teeth
(C) its horny jaws function as do teeth
(D) the horny jaws function as teeth do
(E) the horny jaws function as teeth

In what case, option D could be correct?

Thanks?

The correct idiom is "function as," so we can eliminate A and B. Both C and D can be eliminated for the use of the word "do." If you were comparing the manner in which the horny jaws function to the manor in which the teeth function, "do" would be correct for establishing parallelism. But you're not; what you're actually trying to say is that the horny jaws serve as teeth, or fulfill the function of teeth. So that "do" is unnecessary and confusing. That leaves E as the correct answer.
gmat_perfect, if I understand your question correctly, the only way that I can see for choice D to work is for the function of the teeth to be explained. In that case, "...as teeth do" would have to be followed by a comma and an explanatory modifier. For example, "...as teeth do, grinding up the turtle's food."