Step 1: Recognize correct prepositional usage.
The correct preposition that follows "interested" is in, not "on", "at", or "about".
Step 2: Correct the sentence.
The corrected sentence should be:
"She is interested in learning new languages."
Fill in the blank with the correct option.
The teacher believed that the student’s sudden lack of interest in class was an ..........., as he had always been enthusiastic and attentive.