What is the verb form that means to express agreement?

The verb form that means to express agreement is "assent." This term specifically refers to giving one's approval or agreement to a proposal, idea, or suggestion. It implies not just a passive acceptance but an active endorsement or consent.

While “concur” also relates to agreement, it is often used in contexts where individuals share the same opinion or decision, rather than simply indicating agreement through approval. "Disagree" is clearly contrary to expressing agreement and signifies the opposite of assent. "Comply" suggests following or adhering to rules or requests, which does not directly convey the idea of agreement; it relates more to obedience rather than affirmation. Thus, "assent" is the most precise choice when looking for a verb that denotes the act of agreeing or giving approval.