Which brother offered to stay as a slave in Egypt in place of Benjamin?

Answer: Judah

Judah pleaded, "Please let your servant stay here as my lord's slave in place of the boy. Let him return with his brothers."

“Then Judah approached Joseph and said, “Sir, please let your servant speak personally to my lord. Do not be angry with your servant, for you are equal to Pharaoh himself. Now please let your servant stay here as my lord’s slave in place of the boy. Let him return with his brothers.”— Genesis 44:18, 44:33 (BSB)