Coldline Storage is the most cost-efficient option for this use case because it is designed for data that is accessed infrequently, such as once a quarter or less. It offers the lowest cost per gigabyte compared to other storage options, making it ideal for backup data that is rarely accessed. Nearline Storage, while suitable for less frequently accessed data, has a higher cost per gigabyte than Coldline and is more appropriate for data accessed about once a month. Regional and Multi-Regional Storage options are designed for higher availability and frequent access, resulting in higher costs, and are not the most cost-efficient choices for this scenario.