Ezekiel's first vision came in the fifth year of the exile of which king?

Answer: Jehoiachin

On the fifth day of the month, in the fifth year of King Jehoiachin's exile, the heavens opened and Ezekiel saw visions of God.

“In the thirtieth year, on the fifth day of the fourth month, while I was among the exiles by the River Kebar, the heavens opened and I saw visions of God. On the fifth day of the month—it was the fifth year of the exile of King Jehoiachin—”— Ezekiel 1:1-2 (BSB)