Why C? And why not the others?

Correct answer: C. S3 Glacier Instant Retrieval

AWS documentation describes S3 Glacier Instant Retrieval as designed for long-lived, archive data that's accessed about once a quarter and requires milliseconds retrieval, with the data remaining available for real-time access at any time, which matches this scenario's rare, roughly quarterly access paired with a hard requirement for immediate, dashboard-ready responses. It carries a 90-day minimum storage duration but never requires an explicit restore step before data can be read, unlike the fully archived Glacier tiers. S3 Glacier Flexible Retrieval is explicitly documented as archived and not available for real-time access, requiring an explicit restore that completes in minutes to hours depending on the retrieval tier chosen, which would violate the millisecond compliance-dashboard requirement outright. S3 Glacier Deep Archive is even slower, designed for data accessed less than once a year with restore times measured in hours, making it entirely unsuitable when results must appear in milliseconds. S3 One Zone-IA does offer millisecond access, but it is designed for infrequently accessed data (roughly once a month) rather than the rare, quarterly access pattern described, it is not resilient to the loss of its single Availability Zone, and it costs more than Glacier Instant Retrieval for data accessed this rarely, since its pricing model targets a much higher access frequency.

Source: AWS S3 documentation: Understanding and managing Amazon S3 storage classes — S3 Glacier Instant Retrieval's quarterly-access, millisecond-retrieval design point and 90-day minimum storage duration