Explanation:

The word that needs to be modified is "listened". We need to choose an adverb that describes the manner in which the action of listening is done. Let's analyze each option:

a) Softly: This adverb describes the action of listening in a gentle or quiet manner. It implies that the person listened with a low volume or intensity. However, this may not be the most suitable adverb to modify "listened" as it doesn't convey the idea of paying attention or being cautious.

b) Carefully: This adverb describes the action of listening with attention, caution, and precision. It implies that the person listened in a thoughtful and thorough manner, paying close attention to the details. This is the most suitable adverb to modify "listened" as it matches the requirement of the sentence.

c) Forcefully: This adverb describes the action of listening with force or intensity. It implies that the person listened with a strong determination or effort. However, this adverb doesn't convey the idea of being attentive or cautious, so it is not the best choice to modify "listened".

d) Really: This adverb means truly or genuinely, and it doesn't provide any description of the manner in which the action of listening is done. Therefore, it is not suitable to modify "listened" in this context.

Conclusion:
Out of the given options, the most suitable adverb to modify "listened" is "carefully" as it conveys the idea of listening with attention and caution.