Which of the following solutions BEST complies with laws requiring secure data-at-rest for a critical application while keeping in mind the need for reduced costs?

Installing a new array with hardware encryption disks is a viable solution for data-at-rest encryption but is typically the most expensive option due to the procurement and installation costs of new hardware, which goes against the "reduced costs" requirement.

Enabling encryption on the back-end database is the most direct and often cost-effective way to secure data-at-rest for a critical application. Modern database systems (e.g., SQL Server, Oracle, PostgreSQL, MySQL) offer transparent data encryption (TDE) or similar features that encrypt the database files at rest without requiring expensive hardware upgrades.

IPSec (Internet Protocol Security) is a protocol suite used for securing IP communications by authenticating and encrypting each IP packet of a communication session. It is used for data-in-transit, not for data-at-rest on a storage array.

Enabling HTTPS (Hypertext Transfer Protocol Secure) on the application encrypts data-in-transit between the client and the application server. It does not provide encryption for data stored at rest within the backend database or storage.