Which of the following amino acid will give violet coloured complex...

Neutral ferric chloride ($\text{FeCl}_3$) solution gives a characteristic violet coloured complex with compounds containing a phenolic hydroxyl group.

Among the given amino acids, Tyrosine contains a phenolic $-\text{OH}$ group in its side chain.

Threonine and Serine contain aliphatic $-\text{OH}$ groups, and Cysteine contains a thiol ($-\text{SH}$) group, none of which give a positive neutral $\text{FeCl}_3$ test.

Therefore, Tyrosine will give a violet coloured complex with neutral ferric chloride solution.

Answer: Tyrosine