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Compound B
This is a pale green crystalline solid. When a solution of B is added to sodium hydroxide solution a gelatinous green precipitate is formed. A solution of B produces a white precipitate when added to barium chloride solution. This white precipitate is insoluble in acid. On heating a sample of the solid B a colourless liquid is seen condensing on the upper surfaces of the test tube.
The pale green colour suggests a transition metal salt. A green precipitate with sodium hydroxide solution is produced by iron(II) salts. The white precipitate with acidified barium chloride solution indicates a sulfate. The colourless liquid is most probably water, showing the presence of water of crystallization.
Answer: hydrated iron(II) sulfate.
The iron(II) could have been confirmed by reaction with potassium hexacyanoferrate(III) solution. You can see the thermal decomposition of hydrated iron(II) sulfate here.