The correct option is B under the Cabinet Mission Plan, 1946
The Constituent Assembly was Constituted in November 1946 under the Scheme of Cabinet Mission Plan. The Cabinet Mission consisting of three members- Lord Lawrence, Sir Stafford Cripps, and A.V. Alexander arrived in India on March 1946 and Published the Cabinet Mission Plan on May 1946. The first meeting of Constituent Assembly was held on December 9, 1946.