Which Persian king sent Nehemiah to rebuild Jerusalem?

Answer: Artaxerxes

In the twentieth year of King Artaxerxes, the king granted Nehemiah's request to go and rebuild the city of his fathers.

“Now in the month of Nisan, in the twentieth year of King Artaxerxes, when wine was set before him, I took the wine and gave it to the king. I had never been sad in his presence,”— Nehemiah 2:1 (BSB)