What process would generate a new feature class output of rivers of California from existing feature classes?

The chosen answer is effective because the Clip tool is specifically designed to extract or "clip" features from one layer using the boundaries of another layer. In this scenario, using the USA rivers as the input features and the California state boundary as the clip features allows for the creation of a new output feature class that includes only the portions of the rivers that fall within California’s borders.

This process yields a precise representation of California’s rivers by trimming away any parts of the rivers that lie outside the state boundary, thus producing a focused dataset relevant to California. This is particularly useful for analyses and mapping that require only the geographical area of interest.

In contrast, other options do not appropriately apply the tools for the objective at hand. The Union tool, for example, combines the geometries of two feature classes to create a new one that contains all areas covered by either input. While this could be used to generate a comprehensive dataset, it would not specifically isolate the rivers within California, which is the goal of the exercise.