すみませんが、もう少しゆっくり話し___。
Q. Which is the most polite way to ask 'could you speak more slowly?'
Translation
Excuse me, could you please speak a little more slowly?
Explanation
〜ていただけませんか uses the humble いただく in the potential negative — the most polite request among the choices. 〜てください is a plain polite command, and 〜てあげませんか wrongly casts the listener as doing you a favor you're offering.