The amino acid that gives a red-blood colour on treating its sodium fusion extract with sodium nitroprusside is:

Solution & Explanation

Concept: The sodium nitroprusside test is a confirmatory test for SULPHUR in an organic compound. During sodium fusion (Lassaigne's test) any sulphur in the molecule is converted to sodium sulphide: S + 2Na → Na₂S. The sulphide ion then reacts with sodium nitroprusside, Na₂[Fe(CN)₅NO], to form a deep violet / blood-red complex: Na₂S + Na₂[Fe(CN)₅NO] → Na₄[Fe(CN)₅NOS] (violet-red). So only a sulphur-containing amino acid can give this colour. Scanning the options: leucine, threonine and serine contain only C, H, N, O — no sulphur. Methionine, H₂N·CH(COOH)·CH₂CH₂·S·CH₃, has a thioether (-S-CH₃) group, so it is the only one bearing sulphur. Hence methionine gives the red-blood colouration — answer (C). Trap: do not confuse with cysteine (also S-containing); among the given four, only methionine has S.