Explanation:

- Option 1: begins

- "Begins" is the present tense form of the verb. The phrase "situation turned ugly" suggests a past event, so present tense is not appropriate.

- Option 2: begun

- "Begun" is the past participle form, used with auxiliary verbs like "have" or "had." It cannot stand alone to describe a past action.

- Option 3: beginning

- "Beginning" is the present participle, often used in continuous tenses. It doesn't fit in this straightforward past tense context.

- Option 4: began

- "Began" is the simple past tense form, making it the correct choice here. The sentence "The situation turned ugly as the crowd began to riot" flows correctly in past tense.