The correct option is A Both A and R are true, and R is the correct explanation of A.
The INC did not participate in the in the First Round Table Conference, because many of their leaders, including Gandhi, were imprisoned for their involvement in the Civil Disobedience Movement. When the British realised that the Civil Disobedience Movement had to be withdrawn and INC leaders were necessary for the Round Table Conference, Gandhi and other leaders were released from prison. This was followed by a series of talks between Gandhi and Viceroy Irwin, according to which the British abolished salt taxes and released those who were arrested for taking part in the salt march. The Congress, in return, withdrew the Civil Disobedience Movement and agreed to participate in the Second Round Table Conference.