Observation of "Rhumann's purple" is a confirmatory test for the presence of:

We know from organic reaction theory that the compound called ninhydrin, whose empirical formula may be written as $$C_9H_6O_4$$, reacts strongly with free $$\alpha$$-amino groups. During this reaction the amino acid first undergoes oxidative deamination; carbon dioxide, an aldehyde that has one carbon fewer than the original amino acid, and ammonia are released. The liberated ammonia then reacts further with another molecule of reduced ninhydrin to create a conjugated heterocyclic chromophore that has an intense blue-violet colour.

This characteristic blue-violet chromophore is universally known as “Ruhemann’s purple”. The entire colour producing sequence is therefore called the ninhydrin test. Because an $$\alpha$$-amino group is an integral part of every amino acid residue in a polypeptide chain, the test becomes a general qualitative test for proteins and free amino acids.

Now let us match this analytical fact with the options given in the question. Starch gives an iodine test (blue-black complex), reducing sugars give Tollen’s or Fehling’s brick-red precipitates, and cupric ion is identified by specific colourimetric or precipitation methods. None of these produce the violet chromophore called Ruhemann’s purple. On the other hand, proteins (or any substance containing a primary amino group) definitely produce this colour in the ninhydrin reaction.

So the observation of Ruhemann’s purple is confirmatory evidence for the presence of proteins or amino acids.

Hence, the correct answer is Option C.