Final answer:
A reducing agent, often an electron donor such as zinc or iron(II) ions, reduces exposed silver ions to metallic silver during photographic development.
Step-by-step explanation:
The agent that reduces all exposed, energized, halide crystals to black metallic silver in photographic film is an electron donor, which serves as the reducing agent. In the process of photographic development, exposed silver halide crystals are turned into metallic silver through a reduction reaction, where silver ions (Ag+) gain electrons and are converted into neutral silver atoms (Ag).
The general half reaction for this process is represented as Ag+ (aq) +e− → Ag(s). In this context, the electron donor could be a chemical like zinc or iron(II) ions in various developing solutions used during photo processing.