Choose the option that correctly completes the sentence:

It is mandatory that the proposal _____ submitted before the meeting begins.

The correct option is be.

Explanation:
In English grammar, certain verbs and adjectives that express necessity, requirements, recommendations, or urgency (such as mandatory, essential, crucial, insist, require, recommend) are followed by a clause that uses the subjunctive mood.

The subjunctive mood requires the base form of the verb (also known as the bare infinitive) for all subjects, regardless of tense or person. For the verb "to be", the base form is simply be.

Therefore, in the sentence:
"It is mandatory that the proposal be submitted before the meeting begins."
the verb be correctly completes the structure mandated by the subjunctive mood.