In the context of the input sentence, determine the relationship between 'Banamex' and 'Citigroup'.
Options: parent organization, operator, stock exchange, publisher

Citigroup Inc. on Wednesday confirmed that it will close Banamex USA, a small unit doing business across the U.S.-Mexico border, and agreed to pay $140 million to regulators who said the unit hadn’t done enough to protect itself from money launderers.
operator