Step 1: Work out place A.
The Congress session of September 1920, where Gandhi's Non-Cooperation proposal was accepted, was held at Calcutta, so A is Calcutta.
Step 2: Work out place B.
Gandhi's peasant movement against forced indigo cultivation, the Tinkathia system, was fought in Champaran in Bihar in 1917, so B is Champaran.
Step 3: Note why these places matter.
Calcutta 1920 marked the start of a truly mass movement, while Champaran was Gandhi's first successful satyagraha in India and established his standing as a national leader.
Hence, A is Calcutta and B is Champaran.