Which feature is unique to progressive addition lenses?

Progressive addition lenses (PALs) are designed to provide a smooth transition from distance vision to near vision without any visible lines separating the different areas of prescription. The unique feature of progressive lenses is the gradual increase in power along the vertical gradient of the lens. This allows the wearer to comfortably see at various distances, such as distance, intermediate, and near, without the abrupt shifts characteristic of traditional bifocal or trifocal lenses, which have defined segments or lines.

When considering the other options, a visible segment is typical of bifocal lenses, which indeed have a distinct area for near vision. A fixed distance portion is a feature of single vision lenses or bifocals where the prescription is clearly divided, while a bifocal line explicitly references the separation between the near and distance prescriptions, which is not present in progressive lenses. This gradual change in power is what enables progressive lenses to be more aesthetically pleasing and functional for users needing multifocal correction.