In 1 Corinthians 3, what is the one foundation no one can replace?

Answer: Jesus Christ

No one can lay a foundation other than the one already laid, which is Jesus Christ.

“By the grace God has given me, I laid a foundation as an expert builder, and someone else is building on it. But each one must be careful how he builds. For no one can lay a foundation other than the one already laid, which is Jesus Christ.”— 1 Corinthians 3:10-11 (BSB)