Select the most appropriate option to fill in the blank.
The professor was very happy to see that John was a _______ student.

The context states that the professor was "very happy". A teacher is typically happy to see a hardworking and dedicated student.
Diligent means having or showing care and conscientiousness in one's work or duties (hardworking).

Careless and cunning carry negative or inappropriate connotations for this context, and sunny refers to disposition rather than study habits.

Therefore, diligent fits the blank perfectly.