Question

how does ninhydrin work to develop latent fingerprints?
the dye in it reacts with oils in the fingerprint.
vapors of iodine in ninhydrin react with oils in the fingerprint.
ninhydrin reacts with amino acids in the perspiration of the fingerprint.
its like glue that sticks to the fingerprints.

Explanation:

Ninhydrin is a chemical reagent. Latent fingerprints contain amino acids from perspiration. Ninhydrin reacts with amino acids through a chemical reaction (the Ruhemann's purple reaction). It does not react with oils (so the first two options are wrong). It is not like glue (so the last option is wrong).

Answer:

Ninhydrin reacts with amino acids in the perspiration of the fingerprint.