To ensure the payments processing application can continue to serve requests and avoid user disruption during a regional failure, it's crucial to use a globally distributed database like Cloud Spanner, which is designed for high availability and global consistency. Additionally, to meet the requirement of using AES-256 encryption with control over the encryption key storage, a customer-managed encryption key (CMEK) is necessary. This allows you to manage and control the encryption keys used to protect your data. Option A combines both the use of Cloud Spanner for high availability and CMEK for encryption key control, making it the correct choice. Options B and D do not provide control over the encryption keys, and Option C, while offering CMEK, uses Cloud SQL which is not globally distributed like Cloud Spanner, potentially leading to disruptions in case of a regional failure.