This sentence requires a passive voice construction because the subject, "The assignment," is the recipient of the action, not the performer. Assignments do not submit themselves; they are submitted by someone. When a modal verb (like "must," "can," "should," "will") is used in the passive voice, the structure is modal verb + be + past participle of the main verb.
- The modal verb in this sentence is "must."
- Following the modal, we need "be."
- The main verb is "submit," and its past participle form is "submitted."
Therefore, combining these, the correct phrase is "must be submitted." Option A, "submit," is in the active voice and would imply the assignment performs the action. Option C, "submitted," is a past participle but lacks the auxiliary "be" required for the passive construction with a modal. Option D, "submitting," is a present participle and would be used in an active continuous tense. Thus, "be submitted" is the only grammatically correct and contextually appropriate choice for the passive voice.