Credit card numbers are encrypted when stored in the database but are automatically decrypted when...

The core issue here is cleartext transmission of sensitive data, and option C directly addresses this:

Addressing the Problem: The scenario reveals the vulnerability is the lack of encryption during data transmission (the GET response). Ensuring encryption in transit fixes this specific exploit.

Transport Layer Security: Encryption during transit is typically achieved through protocols like TLS (HTTPS), preventing the interception of sensitive information.