A sounds like a answer but don't let it fool you, same with C. B or D will most likely be your answer I would go with D
Here is the sentence using the words capacity and regulation:
The capacity of the hall can only accommodate at most 500 people; therefore, strict regulation of the flow of the people going in and out must be observed all the time. Regulation means control or management. Capacity means the amount or size that something can contain. Hope this answer helps.
