The correct answer is C (Nearline) because the data is accessed only monthly, making Nearline the ideal choice for storage. Nearline offers a cost-effective solution for infrequently accessed data, balancing lower costs with acceptable availability and access costs. It's particularly suited for backup, disaster recovery, and archival storage where data is accessed once a month or less. For data accessed less than once a year, Coldline would be more cost-effective. Multi-regional and Regional storage options are not optimal for this scenario due to their higher costs and design for more frequently accessed data.