Which chemical substance is used for developing latent prints left by oily substances?

DFO, or 1,8-diazafluoren-9-one, is the correct choice for developing latent prints that are primarily composed of oily substances. It reacts with the amino acids present in the sweat and oils found in fingerprint residue, resulting in a fluorescent compound that makes the prints visible under UV light. This characteristic is particularly valuable in forensic investigations where prints may be faint or difficult to detect with conventional methods.

Other chemical substances like ninhydrin primarily react with amino acids and are effective for developing prints from porous surfaces, but they are not designed specifically for oily residues. Iodine can develop latent prints as well, but it mainly reacts with lipids, and the prints tend to fade quickly unless fixed. Freon 113 was previously used for various applications but is not a recommended choice for developing latent prints due to safety and environmental concerns. Thus, DFO stands out as the most effective choice for developing oily latent prints.