Identify the reaction involved

Ninhydrin reacts with amino acids/proteins to give a characteristic deep purple/violet compound known as Ruhemann's purple.

What is formed in the ninhydrin reaction?

In the reaction, the amino acid undergoes oxidative deamination and decarboxylation. The final colored product is formed by condensation of:

• one molecule of reduced ninhydrin (hydrindantin), and
• one molecule of ninhydrin in the presence of ammonia.

Characteristic structural feature of Ruhemann's purple

Ruhemann's purple is the diketohydrindylidene–diketohydrindamine structure, i.e. a system containing:

• two indane-1,3-dione type rings, and
• a central imino/amino-linked conjugated system responsible for the purple color.

How to identify the correct option

The correct structure must therefore show:

• two ninhydrin-derived ring systems,
• a conjugated linkage,
• nitrogen incorporated in the central chromophore.

Among the given options, this corresponds to Option D.

Conclusion

Therefore, the correct structure of Ruhemann's purple is: $\boxed{\text{D}}$

Comparison with stored answer

Stored correct answer = D.

My derived answer also = D. Hence they agree.