At which step in creating features would you define a river line feature's name?

Defining a river line feature's name is most appropriately done during the process of editing attributes. This step allows you to input and modify the information associated with the feature, which includes naming it. When you edit attributes, you can enter specific data like the feature's name, type, and other relevant characteristics that help describe and classify the feature effectively in your GIS dataset.

While choosing or creating a template is important for determining the default properties and attributes of a new feature, the actual naming occurs when you are in the attribute editing phase, where you directly specify the name alongside other attributes. Similarly, choosing a construction tool and creating feature geometry pertain more to the physical creation and manipulation of the feature itself rather than defining textual attributes like the name. Thus, the most appropriate context for naming the feature is during the editing of its attributes.