Incomplete Sentence

Dear Teacher

When I did some incomplete sentence exercises I found this question that made me confuse:

“________ what to do, he just waited until his father arrived.”

A. Know not
B. Not know
C. No Knowing
D. Not knowing

I know in the sentence, Gerund and Infinitive can be a subject but I am not so sure this question.

Please let me know more about this.

Thank you in advance.

not knowing what to do = as he did not know what to do