The correct answer is A. Silver bromide.
Silver bromide is a chemical compound with the formula AgBr. It is a white, odorless, and tasteless solid that is soluble in water. Silver bromide is used in photography as the light-sensitive material in photographic films and plates. When exposed to light, silver bromide is converted to silver metal, which forms the image on the film.
Hypo is a common name for sodium thiosulfate, Na2S2O3. It is a white, water-soluble solid that is used as a fixing agent in photography. Fixing agents remove unexposed silver bromide from photographic film and paper, preventing it from developing into an image.
Sodium sulfate is a white, water-soluble solid that is used in a variety of industrial and commercial applications. It is not used in photography.
Hydroquinone is a white, crystalline solid that is used as a developing agent in photography. Developing agents convert exposed silver bromide to silver metal, which forms the image on the film.
In summary, the chemical used in developing photographic films is silver bromide. Hypo is a fixing agent, sodium sulfate is not used in photography, and hydroquinone is a developing agent.