Answer:
D. 'These buttons direct incoming calls to the correct people."
Explanation:
A. 'These buttons can be used by you to direct incoming calls to the correct people.'
B. 'If a call is incoming, these buttons can be used to direct it to the correct <u><em>people</em></u>.' (s/b person)
C. 'Directing incoming calls to the correct <u><em>peoples</em></u> (s/b people is) done with these buttons.'
D. 'These buttons direct incoming calls to the correct people."
This is the most acceptable and grammatically correct sentence. The first sentence is correct as well, but the second one is more direct and concise.