Polonius' reason for Hamlet being sad was:
This refers to the old Shakespearean drama which tells the tale of Hamlet who has a thirst for power which led him to commit murders and to take over the throne.
With this in mind, Polonius believes that Hamlet is sad because he has not spoken to Ophelia whom he forbade from speaking to Hamlet because Hamlet was fond of her.
Therefore, the correct answer is option C
Read more about Ophelia here:
https://brainly.com/question/11468942