Assertion (A): The Indian National Congress accepted the Mountbatten plan. Reason (R): It believed in twonation theory.
Let us keep Assertion-Reason simple. In Partition of India and Independence, the right combination here is (A) is true, but (R) is false. First ask, is Assertion true? Then, is Reason true? Then, does Reason explain Assertion? When you answer those three, you get (A) is true, but (R) is false.
Ref: [UPPCS (Pre) 1998] | Bipan Chandra, India's Struggle for Independence, and NCERT History textbooks discuss Partition of India and Independence and explain the events, leaders and movements that support the correct answer for this question.