What is a map key ? what does a map key do?

Which of the following lists shows the events in the correct order? a.)the creation of the british raj, the sepoy rebellion, gandhi’s campaign of civil disobedience, the partitioning of indiab.) gandhi’s campaign of civil disobedience, the sepoy rebellion, the creation of the british raj, the partitioning of indiac.)the sepoy rebellion, the creation of the british raj, gandhi’s campaign of civil disobedience, the partitioning of indiad.)gandhi’s campaign of civil disobedience, the creation of the british raj, the partitioning of india, the sepoy rebellion