Is this museum what you visited?
补语从句（也有叫表语从句的）改回陈述句：this museum is what you visited.
Is this the museum that you visited?
定语从句.改回陈述句：this is the museum that you visited.
Is this museum the one that you visited?
定语从句.改回陈述句：this museum is the one that you visited.