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.

1) their horny jaws function for teeth
2) its horny jaws function for teeth
3) its horny jaws function as do teeth
4) the horny jaws function as teeth do
5) the horny jaws function as teeth

GMAC : 5) totally does away with the possessive pronoun and places an article(?)
Me: 2)

The possessive pronoun actually NEEDS to be done away with, as it has no proper antecedent. The best approximation to an antecedent is 'the turtle,' but that doesn't work because, as used here, 'the turtle' refers to the entire range of turtle species throughout evolutionary history - not just the 'contemporary turtle species' described in the latter part of the sentence.

In any case, the context here makes the possessive unnecessary. Same deal with the following shorter (and easier-to-examine) sentence: In humans, who walk upright, THE spinal cord is perhaps still better adapted to its historical horizontal position.

What is the difference between D and E
I chose D because the horney jaws function as teeth do, making the two verbs to be parallel

Answer should be E. A,B, and C all have possessive pronouns without referring to any possessive noun. Answer choice D is awkward and incorrect, because it ends with "do". Thus we are left with E, which is correct.

but that's not what the sentence means.
the intended meaning is that the horny jaws play the role of teeth, which is the correct interpretation of 'function as teeth'.
i.e.,
function as teeth --> they work as an acceptable substitute for teeth
function as teeth do --> they work in the same way as teeth do - same mechanics, same action, etc. this is not what is meant.