Gandhiji launched the Non-cooperation movement on 5th Sept 1920 with the aim of self-governance and obtaining full independence. So the assertion is true. But then the movement was launched after the Jallianwala Bagh massacre took place on 13 April 1919. Therefore, actually the Non-cooperation movement was a retaliation to the Jallianwala Bagh massacre. So, the reason is false. Hence, "A is true but R is false." is the correct answer.