People were prohibited _______ their vehicles near the entrance of the main administrative building.

The correct answer is from parking.

Explanation:

In English grammar, certain verbs are followed by specific prepositional patterns. The verb prohibit follows the structure:

prohibit someone/something + from + gerund (-ing form of the verb)

Let's analyze why the options work or do not work:

1. from parking: Correct. It follows the standard pattern prohibit + object + from + verb-ing.

2. to park: Incorrect. The verb "prohibit" does not take an infinitive ("to" + verb) directly following its object.

3. to have parked: Incorrect. The perfect infinitive is grammatically incorrect after "prohibit".

4. parking: Incorrect. It misses the required preposition "from".

Therefore, inserting from parking completes the sentence correctly: "People were prohibited from parking their vehicles near the entrance of the main administrative building."