Which one is not a modal verb?

Modal verbs are a type of auxiliary verb that express necessity, possibility, permission, or ability. They always precede a main verb and do not change form based on the subject, unlike regular verbs.

• The correct option is be. "Be" is a primary auxiliary verb or a linking verb, not a modal verb. It conjugates extensively (e.g., am, is, are, was, were, being, been) and can stand alone or be followed by a main verb in progressive or passive constructions. Its varied forms and ability to function as a main verb distinguish it from true modals.
• Distractors: "Can" and "could" are quintessential modal verbs, expressing ability, permission, or possibility. Therefore, "all" is incorrect as these are clearly modals.