Match List-I with List-II.
List-I (Element detected)   List-II (Reagent used/Product formed)
A. Nitrogen                         I.      Na$$_2$$FeCN$$_5$$NO
B. Sulphur                           II.     AgNO$$_3$$
C. Phosphorus                   III.    Fe$$_4$$FeCN$$_{6_{3}}$$
D. Halogen                          IV.   NH$$_{4_{2}}$$MoO$$_4$$

In qualitative analysis of organic compounds (Lassaigne's test):

A. Nitrogen detection: The sodium fusion extract containing NaCN is treated with FeSO₄ and then with dilute H₂SO₄. The formation of Prussian blue colour confirms nitrogen. The product is Fe₄[Fe(CN)₆]₃ → Match with III.

B. Sulphur detection: The sodium fusion extract containing Na₂S is treated with sodium nitroprusside Na₂[Fe(CN)₅NO]. A violet colour confirms sulphur → Match with I.

C. Phosphorus detection: The compound is heated with an oxidizing agent, dissolved in HNO₃, and treated with ammonium molybdate (NH₄)₂MoO₄. A yellow precipitate of ammonium phosphomolybdate confirms phosphorus → Match with IV.

D. Halogen detection: The sodium fusion extract is acidified with HNO₃ and treated with AgNO₃. The formation of a precipitate (AgCl, AgBr, or AgI) confirms halogens → Match with II.

The correct matching is: A → III, B → I, C → IV, D → II.