I must do it. (make it negative)

A I cannot but doing this.

B I can help do this.

C I cannot but did this.

D I cannot help doing this.

Correct Answer: Option D

Affirmative বাক্যে must থাকলে negative করার সময় must এর পরিবর্তে cannot but + verb এর base form / cannot help + verb + ing বসে।