What chemical compound is primarily used in physical developers?

Silver nitrate is the correct answer because it serves as the main chemical compound in physical developers, particularly in forensic applications where latent fingerprints need to be enhanced. This compound is effective because it reacts with the organic components found in fingerprint residues, forming a visible dark brown or black stain that can then be photographed or otherwise documented.

In this context, silver nitrate's ability to form a precipitate upon reaction with halides present in the fingerprints makes it particularly valuable for visualizing prints on surfaces that may not yield clear impressions through other means.

While iron oxide, copper sulfate, and sodium chloride have their uses in various chemical processes, they are not typically employed as primary agents in physical development for fingerprints. Iron oxide is commonly used in other applications, copper sulfate is often used in agricultural practices or as a fungicide, and sodium chloride primarily has roles in biological and nutritional fields. These other compounds do not provide the same clarity or efficacy in visualizing latent fingerprints as silver nitrate does.