What is the relationship between 'Banamex' and 'Citigroup' in the context of the input sentence.
Options: operator, parent organization, chairperson, 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