On his sudden demise, my emotions were so, complicated that it was ______ how I felt.

The correct answer is: D. unimaginable.

The other options are incorrect because:

• A. unreasonable: It is not unreasonable to feel complicated emotions after the sudden death of a loved one.
• B. impossible: It is possible to feel complicated emotions after the sudden death of a loved one.
• C. intolerable: It is not intolerable to feel complicated emotions after the sudden death of a loved one.

The word “unimaginable” means “too great or extreme to be imagined.” In this context, it means that the speaker’s emotions were so complex and overwhelming that they were difficult to describe or understand. This is a common experience for people who have lost a loved one suddenly.