Which setting only allows for changes in how a label appears, not its placement?

The correct choice pertains to the "Effects" setting within labeling options in ArcGIS Pro. This setting is specifically designed to modify visual aspects of how labels appear, such as their color, font style, size, and shadow effects. These adjustments enhance the aesthetic presentation of labels on a map without impacting their position or spatial relationship to the features they annotate.

This is different from the other options. Weights, for instance, are related to how labels compete for space and can influence their placement based on the importance given to them. Scale settings are employed to control when labels become visible or how they are displayed at different map scales, which directly ties into their placement context. Classes allow for defining groups of features that can share similar label properties, affecting how labels are managed collectively, but they can also involve placement rules depending on those classes. Therefore, "Effects" stands out as the setting focused solely on appearance rather than placement.