B. By using an object standard set controller action

• This approach involves creating a PageReference by specifying the action associated with a standard set controller. For example:
PageReference pageRef = new PageReference('/apex/YourPageName');

C. By using ApexPages.currentPage()

• This method returns a PageReference object that refers to the current Visualforce page. For example:
PageReference currentPage = ApexPages.currentPage();

Correct answers: C, D
https://developer.salesforce.com/docs/atlas.en-us.apexref.meta/apexref/apex_system_pagereference.htm