Take context from the input text and decide on the accurate relationship between 'Banamex' and 'Citigroup' from the options provided.
Options: parent organization, chairperson, operator, headquarters location

Citigroup has faced a number of probes over Banamex after it announced a charge of about $235 million in February 2014 due to uncovering "significant" fraud on the books of oil contractor Oceanografia after having lent it $585 million through a Banamex unit.
operator