Improve the bracketed part of the sentence.
The naughty boy made (many a mischief).

The word mischief is an uncountable noun and cannot be used with countable determiners like "many" or "many a" in standard English.
To express a large amount of an uncountable noun, we use much.

Therefore, "many a mischief" should be replaced with much mischief.

Hence, option A is the correct improvement.