This question isn't in the imperative and is therefore misleading to a student.

Could "Parla lui !" give some emphasis to HE, ? should it be included as there is an exclamation mark

Why was "Dice" wrong?

This is a trick question. In English, 3rd person imperative is "let him speak" and the appropriate Italian is "parli". However, this is a 3rd person indicative [exclamation!] and the Italian is "parla".