What capability does a 3D RNAV system have in relation to navigation?

A 3D RNAV (Area Navigation) system is designed to provide navigation capabilities in both horizontal and vertical planes simultaneously. Thus, the correct answer emphasizes its ability to manage navigation across multiple dimensions.

The system allows aircraft to navigate seamlessly along a pre-defined route, taking into account both lateral (horizontal) and vertical (altitude) navigation components. This means that it can guide an aircraft not only along its flight path but also assist in maintaining the desired altitude, which is critical for efficient and safe flight operations.

In contrast, focusing solely on vertical navigation neglects the comprehensive functions that 3D RNAV systems offer, which include managing both lateral navigation and vertical profiles for optimal flight performance.