Select the most appropriate option to fill in the blank.
It was impossible ______ guess what he was doing.

Correct option is A

The correct option to fill in the blank is (a) to.
· The word "to" is used before the base form of a verb (infinitive) to indicate purpose or action. Here, "to guess" is the correct infinitive form following the adjective "impossible". (Hindi: को)
· Example: It was impossible to understand what he was saying.
Explanation of incorrect options:
· At (Hindi: पर): Used for location, time, or reference, incorrect here.
· On (Hindi: पर): Used for position or a specific day, does not fit grammatically.
· Of (Hindi: का/की): Used for possession, not appropriate in this structure.