What should an ArcGIS user do to ensure the exact outline of all building and parcel features is visible?

Setting transparency on buildings is the correct approach to ensure that the exact outline of all building and parcel features is visible. By adjusting the transparency settings, users can make the building polygons partially see-through, which allows the underlying parcel outlines to show through clearly. This is particularly useful in situations where the building features might otherwise obscure important details of the parcel boundaries.

The other options do not effectively achieve the objective of visual clarity for both building and parcel outlines. Spatially joining features could create a new dataset that combines building and parcel information, but it would not necessarily enhance visibility; it could complicate the visual representation. Joining buildings to parcels may associate the attributes of buildings with parcels but does nothing to improve the visual clarity. Relating buildings to parcels provides a connection for data analysis purposes but does not impact how features are visually displayed on the map.