A pale yellow substance (A), when heated with conc. `HNO_(3)` , produces a brown gas (B).

Explanation

These observations suggest that the substance (A) is sulphur (S), the gas (B) is nitrogen dioxide `(NO_(2))` , compound (C) is sodium thiosulphute `(Na_(2)S_(2)O_(3))` and the gase (D) is sulphur dioxide `(SO_(2))` . The reaction are as follows :
`S(A) + 6HNO_(3) to H_(2)SO_(4) + 2H_(2)O + 6NO_(2)(B)`