In the context of the input sentence, determine the relationship between 'Banamex' and 'Citigroup'.
Options: parent organization, chairperson, headquarters location, operator

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