He was always prepared to refute the idea.

Answer: Option D

Refute : prove (a statement or theory) to be wrong or false; disprove.
Agree : have the same opinion about something; concur.
Subscribe : arrange to receive something, typically a publication, regularly by paying in advance.
Recommend : put forward (someone or something) with approval as being suitable for a particular purpose or role.
Endorse : declare one's public approval or support of.

Antonym of Refute is Endorse