Which method is used to reveal latent fingerprints on non-porous surfaces that are greasy?

Sudan Black is a dye used specifically for revealing latent fingerprints on non-porous surfaces, especially those that are greasy or oily. The reason for its effectiveness lies in its ability to bind to the lipids present in the sweat residue left behind by fingers. When applied to a greasy surface, Sudan Black permeates the oils, providing a contrasting color that makes the fingerprint patterns visible. This method is particularly well-suited for surfaces where other reagents, such as Ninhydrin or Silver Nitrate, might not work effectively due to the presence of grease, which can hinder their ability to interact with amino acids or chloride ions in the fingerprint residue.

While the other methods have their specific applications—like Ninhydrin for porous surfaces, Silver Nitrate for surfaces where chloride is present, and Physical Developer for a wide range of situations, including wet conditions—they are not ideal for surfaces that are predominantly greasy. Thus, the effectiveness of Sudan Black in revealing fingerprints under these conditions makes it the correct choice for this scenario.