This question tests your understanding of verb tenses and voice in English grammar. Let's analyze the sentence structure:

The sentence states: "The reports ___________________ by four o'clock because the manager is going to sign them."

Key observations:

1. The subject is "the reports" - this is the thing being acted upon (passive voice)

2. There's a deadline: "by four o'clock" indicating a future time constraint

3. The reason given uses future tense: "is going to sign them"

Step-by-step analysis:

Step 1: The reports are the object of the action (being finished), so we need passive voice

Step 2: The deadline "by four o'clock" suggests obligation with future reference

Step 3: "Must" expresses strong obligation or necessity

Step 4: The passive form of "must finish" is "must be finished"

Why the other options are incorrect:

"Had to be finished" - past obligation, doesn't match future context

"Must finish" - active voice, but reports can't finish themselves

"Should have been finished" - past expectation with present consequence, doesn't fit future deadline

Final answer: The correct option is "Must be finished" because it expresses future obligation in passive voice, matching the sentence structure and time context.

Related Grammar Concepts

Modal Verbs: Words like must, should, have to that express necessity, obligation, or possibility

Passive Voice: When the subject receives the action rather than performs it (formed with "be" + past participle)

Future Time Reference: Expressions like "by four o'clock" that indicate a future deadline or time constraint

Key Formulas/Rules

Modal + be + past participle = Passive voice with modals

Example: must + be + finished = must be finished

Subject (receiver) + modal + be + past participle + time reference