Question

The earth is round.(Chane the voice)​

Answer 1

It is intransitive sentence. Therefore it can't be change in passive voice.

Answer 2

"They said that the earth is round" is this sentence active voice or passive voice?

It is an usual example of Direct and Indirect speech chapter. You question looks a bit confused one in asking .

The usual pattern of question runs as:

They said, “The earth is round.”

Change into Indirect speech.

Answer : They said that the earth is round.

Note : Tense of the reported speech is not changed as it is an universal truth statement.

Any how , for your satisfaction , it is in active voice not passive voice sentence