A multi-region Cloud Spanner instance is designed for the highest availability and geographic resilience. It synchronously replicates data across multiple regions, ensuring that if an entire region becomes unavailable, the instance can automatically and transparently fail over to another region. This architecture provides a Recovery Point Objective (RPO) of zero, as all committed transactions are present in other regions. The Recovery Time Objective (RTO) is measured in seconds, as the failover is automatic. This configuration is the only one that meets the aggressive sub-5-minute RTO and RPO requirements while surviving a complete regional failure.

B. A single-region instance represents a single point of failure at the regional level. Daily backups result in an RPO of up to 24 hours, which drastically fails the requirement.

C. Read-only replicas cannot accept writes. In a regional failure, the application would lose write capability, failing to meet the RTO for a fully operational e-commerce database.

D. Multi-region Spanner achieves its high availability and zero RPO through synchronous replication using the Paxos consensus algorithm, not asynchronous replication, which would introduce potential data loss.

1. Google Cloud Documentation - Regional and multi-region configurations: "Multi-region configurations allow Spanner to replicate the database's data in multiple regions... In the event of a single region failure, a multi-region instance can transparently continue to serve reads and writes... RPO (Recovery Point Objective) is 0... RTO (Recovery Time Objective) is on the order of seconds." This directly supports the choice of a multi-region instance for near-zero RTO/RPO.

2. Google Cloud Documentation - Build high availability and disaster recovery readiness with Cloud Spanner: "For the lowest RTO and RPO, use a multi-region instance configuration. This configuration provides the highest availability (99.999%) and protection against regional failures." This document explicitly recommends the multi-region configuration for the scenario described in the question.

3. Corbett, J., Dean, J., Epstein, M., et al. (2012). Spanner: Google's Globally-Distributed Database. In Proceedings of the 10th USENIX Symposium on Operating Systems Design and Implementation (OSDI '12). Section 4, "Implementation," describes the use of the Paxos consensus algorithm across replicas in different datacenters to ensure data consistency and availability. This foundational paper explains the synchronous replication mechanism that enables the zero RPO characteristic of multi-region Spanner. (DOI: Not available for the original conference paper, but widely accessible via ACM/USENIX).