'I must do it' negative form is-

A I cannot but doing this.

B I cannot help do this.

C I cannot do 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/cannot but + verb + ing বসে।