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.
Refer to the input text as context and select the correct relationship between 'Banamex' and 'Citigroup' from the available options.
Options: chairperson, operator, parent organization, headquarters location