Final answer:
In the given context, the word 'boisterous' means 'rowdy', indicating noisy and uncontrolled behavior by the children when their parents left. The correct answer is option b.
Step-by-step explanation:
The meaning of the word boisterous in the context of the sentence 'The children seemed quiet and angelic, but when their parents left they quickly became BOISTEROUS' is 'B. rowdy'.
To be boisterous is to be noisy, energetic, and cheerful in a loud or lively way.
The children were quiet when their parents were around, but their behavior changed to being lively and uncontrolled once their parents left, indicating that 'rowdy' is the correct answer from the given options.