Which apostle did King Herod have put to death with the sword in Acts 12?

Answer: James, brother of John

Herod had James, the brother of John, killed with the sword; seeing this pleased the Jews, he also seized Peter.

“About that time, King Herod reached out to harm some who belonged to the church. He had James, the brother of John, put to death with the sword. And seeing that this pleased the Jews, Herod proceeded to seize Peter during the Feast of Unleavened Bread.”— Acts 12:1-3 (BSB)