Step 1: Understand the grounds for divorce under Section 13 of the Hindu Marriage Act, 1955.
Section 13 of the Hindu Marriage Act lists the grounds on which either party to a marriage may seek divorce.
These grounds include adultery, desertion, and conversion to another religion.
Step 2: Identify the exception.
Insanity is not directly listed as a ground for divorce under Section 13.
Though insanity can affect marital relations, it is not one of the specified grounds under the Act.
Step 3: Conclusion.
Therefore, the correct answer is (D) Insanity.