Can Ajax call redirect a user to the login page from the Controller in MVC?

yes,can redirect

Yes ,you can ...

We can just call ajax as a Anonymous user

yes we can redirect to the login page by using Ajax call

Suppose I have a call which load partial view in a popup using ajax. After session expire we load login page. But the issue is login page open in popup while it should redirect on login page.

Yes obviously, By making ajax call we can redirect to any controller(not saying about authentication).