Who was anointed by one of Elisha's prophets to be king of Israel?

Answer: Jehu

One of the prophets poured oil on Jehu's head and declared him king over Israel.

“So Jehu got up and went into the house, where the young prophet poured the oil on his head and declared, “This is what the LORD, the God of Israel, says: ‘I anoint you king over the LORD’s people Israel.”— 2 Kings 9:6 (BSB)