Testing whether a website supports multiple concurrent users signed in at the same time is a question about system behavior under load/concurrency. Among the listed methods, stress testing is the best match because it evaluates how the system performs when pushed toward or beyond normal operating conditions—often involving high volumes of transactions or many simultaneous users/sessions. CompTIA Project+ explicitly includes stress testing as one of the testing cycles/methods candidates should understand in the context of quality and performance management.

The other choices don’t align: unit testing validates individual components/functions in isolation, not full end-to-end multi-user session handling; smoke testing is a quick confidence check that basic functionality works after a build/change; and regression testing verifies that existing features still work after changes—useful, but not primarily focused on concurrent-user capacity.

So, to answer the PM’s specific question (concurrent sign-ins), the team should run a stress test (often implemented using load-generation tools) to validate session handling, authentication, and resource utilization when many users are logged in simultaneously, and to identify bottlenecks or failure points.