Select the most appropriate option to fill in the blank.

It was impossible ______ guess what he was doing.

to

The most appropriate option to fill in the blank is:

☀ to

The sentence would read: "It was impossible to guess what he was doing."

Explanation: "To" is the correct preposition to use before the verb "guess" in this context. The sentence structure requires the infinitive form of the verb after "impossible." The other options don't fit the context:

• At: This preposition doesn't fit with "impossible to guess."

• On: This preposition doesn't fit with "impossible to guess."

• Of: This preposition doesn't fit with "impossible to guess."