During which step of electrophotographic printing does the corona wire send negative ions to the imaging drum?

The step of charging is critical in electrophotographic printing as it establishes the initial conditions necessary for the process to work effectively. During this phase, a corona wire generates negative ions that are applied to the imaging drum. The negativity of these ions causes the surface of the drum to become negatively charged.

This charging is essential because it prepares the drum's surface to attract positively charged toner particles in the subsequent steps of the process. By having a uniform charge across the imaging drum, it ensures that the toner can be applied accurately and forms the desired image when the drum is later exposed to light.

In contrast, the other steps—fusing, cleaning, and exposing—serve different purposes. Fusing involves melting the toner onto the paper; cleaning is about removing any residual toner from the drum; and exposing pertains to transferring the image onto the drum via a light source. These steps do not involve the initial charging of the drum, which is why charging is the correct answer in this context.