Who is Pedro most likely addressing with this command? ¡Hable más lentamente, por favor!
A.
his friend
B.
his parents
C.
his teacher
D.
his classmates

I believe that it is C. his teacher

Explanation:

The translation: "¡Hable más lentamente, por favor!" Translates into: "Speak more slowly, please!"

Talking in this way could more likely be towards his teacher, if his teacher is teaching and he is asking them to speak slower so Pedro can better understand.