The correct answer is D: The region where the shape of the radiation pattern no longer varies with distance. The far field of an antenna is the region where the shape of the radiation pattern no longer varies with distance. Beyond this distance, the pattern is stable and predictable. The far field (also called Fraunhofer region) begins at a distance where the radiation pattern has stabilized. In the far field, the pattern shape is constant regardless of distance - only the signal strength decreases with distance. The near field is closer, where the pattern shape changes with distance. The far field typically starts at distances greater than 2D²/λ, where D is the antenna's largest dimension and λ is wavelength. Far field measurements are used for antenna testing because the pattern is stable and representative of the antenna's actual performance.