Correct Answer: Considerate

Solution : The right option is the third option .

Nonchalant means showing a lack of concern, whereas considerate means showing concern for the feelings or needs of others. So, "considerate" is the antonym that best opposes the meaning of "nonchalant".

The meanings of other given words are:

• Boisterous : Someone who is noisy, energetic, and rowdy, often characterised by high-spirited or exuberant behaviour.
• Unconcerned : Someone who is not worried or bothered; lacking interest or involvement in a particular situation.
• Retrospective : That means looking back on or directed to the past; contemplating or reviewing past events or situations.

Therefore, the correct answer is considerate .