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.