They say that the earth is round.

It has been said that the earth is round.
That the earth is round is said by them.
It is said that the earth is round.
It was said that the earth was round.

The correct answer is C. It is said that the earth is round.

A. It has been said that the earth is round. This is incorrect because it uses the past tense, which is not appropriate for a statement that is still true today.

B. That the earth is round is said by them. This is incorrect because it uses the passive voice, which is not necessary in this case.

D. It was said that the earth was round. This is incorrect because it uses the past tense, which is not appropriate for a statement that is still true today.

The statement “They say that the earth is round” is in the present tense, so the correct answer should also be in the present tense. Option C is the only option that is in the present tense and is grammatically correct.