User A should have application permissions that are identical to User B's permissions.

To grant User A access to the "My Cases" page, which User B has, the most effective method would be:

C. Update the User A account to access the same portal as User B. This solution ensures that User A has the same structural access as User B, including all the pages and capabilities available in the portal. The portal configuration typically includes access to various pages like the "My Cases" page.

Options A, B, and D do not specifically ensure that User A gains access to the "My Cases" page as they do not address portal-level access settings.