Final answer:
Setting 'Selective Availability' to zero on a GPS unit increases its accuracy to the level of Precise Positioning Service by removing the intentional error introduced into the GPS signal.
Step-by-step explanation:
The feature on a GPS unit that, when set to zero, might allow it to have an accuracy equivalent to Precise Positioning Service receiver capability is c) Selective Availability. This was a deliberate degradation of the signal implemented by the U.S. Department of Defense to prevent adversaries from using highly accurate GPS signals. When Selective Availability is set to zero, the intentional error introduced into the GPS signal is removed, allowing for more accurate positioning. It's important to understand that satellite geometry refers to the relative positions of the satellites in the sky, which affects accuracy but is not a deliberate feature. Ephemeris data is crucial for determining the precise orbit of a satellite and is not degraded on purpose. Differential corrections are enhancements used in conjunction with GPS to improve accuracy by correcting signal errors.