In Ezekiel 9:4, who received a mark on their foreheads?

Answer: Those groaning over abominations

The man clothed in linen was to mark the foreheads of those sighing and groaning over all the abominations committed in Jerusalem.

““Go throughout the city of Jerusalem,” said the LORD, “and put a mark on the foreheads of the men sighing and groaning over all the abominations committed there.””— Ezekiel 9:4 (BSB)