In which year of Solomon's reign did he begin building the temple?

Answer: Fourth year

Solomon began building the temple in the fourth year of his reign.

“In the four hundred and eightieth year after the Israelites had come out of the land of Egypt, in the fourth year of Solomon’s reign over Israel, in the month of Ziv, the second month, he began to build the house of the LORD.”— 1 Kings 6:1 (BSB)