Explanation

The first and third Round Table conferences did not take part in the Indian National Congress.

*First round table conference: 1930 and January 1931 in London
The Indian National Congress decided not to participate
As Many of the INC leaders were imprisoned due to their involvement in the civil disobedience movement.

*3rd round table conference: 17 Nov 1932 and 24 Dec 1932
The Indian National Congress was not invited to this table conference 
Because the recommendations of this conference were published in a White Paper in 1933.

Additional informational 
The Round Table Conference was a series of Three Conference 
Conducted by the Labour Party-led British government 
To discuss constitutional reforms in British India during 1930-32
The second Round Table conference was held from 7 Sep 1931 to 1 Dec 1931