Where else, besides user stories and deployments, can Apex tests be executed?

The correct choice is sandbox environments. Apex tests can be executed in Salesforce within sandbox environments, which are specifically designed for development and testing purposes. Sandboxes allow developers to run their tests in an isolated environment that mirrors the production environment, ensuring that code behaves as expected without affecting live data.

In this context, executing Apex tests in sandbox environments enables teams to evaluate the functionality and performance of their code changes before deploying them to production. This testing is crucial for maintaining code quality and ensuring that new features do not introduce bugs or regressions.

Other options do not provide suitable environments or scenarios for running Apex tests. API calls refer to interactions with Salesforce through external systems, which typically don't involve executing tests directly. Credentials generally involve user authentication and authorization and have no direct correlation with executing Apex tests. Org configurations relate to the overall setup of a Salesforce organization but do not serve as a platform to run tests.