Neutral ferric chloride (FeCl3) solution gives a characteristic violet coloured complex with compounds containing a phenolic hydroxyl group.
Among the given amino acids, Tyrosine contains a phenolic −OH group in its side chain.
Threonine and Serine contain aliphatic −OH groups, and Cysteine contains a thiol (−SH) group, none of which give a positive neutral FeCl3 test.
Therefore, Tyrosine will give a violet coloured complex with neutral ferric chloride solution.
Answer: Tyrosine