A) Well-off
B) Well-advised
C) Well-balanced
D) Behave well

The correct answer is option D - "Behave well".

Explanation:
Adverbs are words that modify or describe verbs, adjectives, or other adverbs. They provide additional information about how, when, where, or to what extent an action is performed. In this given set of options, the only word that functions as an adverb is "well", and it is used to describe the manner in which someone behaves.

Let's break down the options:

A) Well-off: The word "well-off" is an adjective phrase that describes someone who is wealthy or financially comfortable. It does not modify a verb, adjective, or another adverb, so it is not an adverb.

B) Well-advised: The word "well-advised" is an adjective phrase that describes someone who has received good or wise advice. Similar to option A, it is not an adverb because it does not modify a verb, adjective, or another adverb.

C) Well-balanced: The word "well-balanced" is also an adjective phrase that describes something that is properly proportioned or harmonious. It does not function as an adverb because it does not modify a verb, adjective, or another adverb.

D) Behave well: The word "well" in the phrase "behave well" is an adverb. It describes the manner in which someone behaves. It modifies the verb "behave" and provides additional information about how the action is performed.

Therefore, the only option that contains an adverb is option D - "Behave well".