Which pillar of the AWS Well-Architected Framework does this goal represent?

Explanations:

The goal of allowing customers to use the app without upgrading their mobile devices is not directly related to security. Security focuses on protecting data, networks, and applications, not device compatibility.

Reliability ensures that the app is consistently available and works properly, but it does not specifically address the goal of ensuring compatibility with existing devices without upgrades.

Cost optimization involves reducing costs by choosing efficient resources. This does not directly address ensuring the app works on existing devices without upgrades.

Sustainability in this context refers to ensuring the app is usable on a wide range of devices without requiring upgrades, thus promoting longer device lifespans and reducing the need for new hardware.