The correct option is C The Congress boycotted the 1st Round Table Conference.
The Congress which was unhappy with the report of the Simon Commission so, congress boycotted the 1st round table conference. while other political parties participated in 1st Round table conference.
Following are corrected statements :-
1) Gandhi-Irwin pact was signed on 5th March 1931 which happened after 1st Round Table conference.
2) Muslim league participated in all three round table conferences.
3) Congress boycotted 3rd Round Table conference.