Question

1. I knew that Mercury (be) the closest planet to the sun, but I didn’t feel like answering the question.

Answer 1

That Mercury is the closest

Answer 2

The correct form of the verb to complete the sentence is "is" as follows:

"I knew that Mercury is the closest planet to the sun, but I didn't feel like answering the question."

This sentence is in the past tense and the verb "is" is the correct conjugation of "to be" for the third person singular subject "Mercury" in the present tense.