He declined to speak about the problem when I asked him.

Answer: Option C

Agreed : discussed or negotiated and then accepted by all parties.
Promised : assure someone that one will definitely do something or that something will happen.
Refused : indicate or show that one is not willing to do something.
Permitted : officially allow (someone) to do something.
Declined : politely refuse (an invitation or offer).

Synonym of Declined is Refused