Explanation:

- Option 1: Withdrawal of support by Congress

- Congress did withdraw support after the Chauri Chaura incident, but this alone didn’t end the Khilafat Movement.

- Option 2: Abandonment of the policy of discrimination against the Muslims by the British

- The British did not really abandon discrimination against Muslims during that time.

- Option 3: Reservation of seats for Muslims in Central and Provincial legislatures under the Act of 1919

- This happened, but it was not the main reason for the end of the Khilafat Movement.

- Option 4: Assumption of power by Mustafa Kemal Pasha in Turkey and abolition of the Caliphate

- This was the crucial event. Mustafa Kemal Pasha led reforms in Turkey and finally abolished the Caliphate in 1924, making the cause of the Khilafat Movement irrelevant.