He is greatly admired for his . . . . . . behaviour.

The correct answer is D. decorous.

Decorous means behaving or done in a way that is proper or polite. It is the opposite of impolite or rude.

Decrepit means in a state of decay or disrepair. It is the opposite of healthy or well-maintained.

Decorative means serving to decorate or embellish something. It is the opposite of plain or simple.

Decadent means characterized by moral or cultural decline. It is the opposite of virtuous or wholesome.

In the context of the question, the person is being admired for their proper or polite behavior. Therefore, the correct answer is D. decorous.