Which of the following amino acid will give violet coloured complex with neutral ferric chloride solution?

Neutral ferric chloride gives violet coloured complex with compounds containing a phenolic $$−OH$$ group.

Among amino acids, Tyrosine contains a phenolic $$−OH$$ group in its side chain.

Structure of Tyrosine:

$$OH-C_6H_4-CH_2-CH\left(NH_2\right)-COOH$$

Tyrosine$$+$$ $$FeCl_3$$ $$\longrightarrow\ $$Violet Coloured Complex

Serine and Threonine: These amino acids contain an aliphatic/alcoholic hydroxyl group$$\left(-OH\right)$$.

Cysteine: This amino acid contains a sulfhydryl group $$\left(-SH\right)$$

Therefore, the amino acid which gives violet coloured complex with neutral ferric chloride solution is Tyrosine.

Option C  is correct.