The committee has submitted its report .. the students' riot.

Analyze the question: This tests your knowledge of prepositions, specifically which one correctly follows "report" when we mean the subject matter of the report.

Explain the concept: When "report" is followed by the topic or subject it deals with, the standard preposition is on.

• A report on the economy
• A report on the accident
• A report on the students' riot

"About" is more casual and general. "On" is the formal, standard choice for official documents such as reports, studies, and investigations, which is why examiners prefer it.

Step-by-step solution:

1. The sentence says the committee submitted its report. We need a preposition that links "report" to its subject, which is the students' riot.
2. Test each option: • A. on: "report on the students' riot" is correct and idiomatic. • B. for: "report for" suggests the report is meant for someone or something, not that it is about the riot. This does not fit. • C. about: Not completely wrong in everyday speech, but it is less formal and not the standard collocation for an official committee report. • D. at: "report at" is incorrect here.
3. The best answer is on.

Answer: A. on

Exam tip: Remember the pattern "report on, research on, inquiry into, investigation into." Formal documents that examine a topic usually take on. If the sentence involves a committee, a panel, or an official report, think "on" first.