Although the turtle has been toothless for more than 150 million years

This problem tests knowledge of idioms and meaning (fundamentally)

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

I wasn't particularly sure about whether turtle species should be accompanied by the plural their or its here so I looked to other points.

C - The placement of do prior to teeth here makes the sentence read as though the horny jaw functions as do teeth almost like conveying "so does X" e.g. John smokes, so does Ashley That's not what the sentence is trying to say
D - This sentence is literally saying 'the horny jaws function as teeth function'; a comparison between functions
E - Correct; what we are trying to say (and this is tipped off by the subordinate clause at the beginning which gives context) is that the moderately sharp and jagged edges of the horny jaw function AS teeth