The blank precedes the noun, "student". Hence it has to be filled in by an adjective, which describes "students who never break rules".
Option A: The word, "mischievous" is an adjective which means ‘causing trouble’. Mischievous students break the rules. Hence, the word does not describe students who never break rules and this option is incorrect.
Option B: The word, "studious" is an adjective which means: Spending a lot of time studying or reading. It is not necessary that a studious student does not break the rules. Hence, this option is incorrect.
Option C: The word, "stubborn" is an adjective which means: Having or showing a strong determination to not change one's attitude or position on something, especially in spite of good reasons to do so. Being stubborn has no connection with not break rules. Hence, this option is incorrect.
Option D: The word, "obedient" is an adjective which means: Complying or willing to comply with an order/ rule. An obedient person will comply with rules i.e he never breaks rules. Hence, this option is the correct answer.