Select the most appropriate option to fill in the blank.
I looked at her face and found she was getting disturbed. I decided to drop that
________.

Correct option is C

The most appropriate option to fill in the blank is (c) subject.
 The sentence suggests that the speaker observed a change in the listener's facial expression and realized that the topic they were discussing was causing her distress.
 Therefore, the speaker decided to drop the subject, meaning to stop discussing that particular topic to avoid causing further discomfort to the listener.