Final answer:
When an inferior planet reaches its greatest angular separation from the Sun, it's at aphelion, the farthest point in its elliptical orbit around the Sun.
Step-by-step explanation:
When an inferior planet is as far from the Sun (in angular separation) as it ever gets, the planet is said to be at aphelion. This term specifically refers to the point in a planet's elliptical orbit around the Sun where it is at its greatest distance from the Sun. By contrast, the term perihelion refers to the closest point to the Sun in a planet's orbit.
This concept is part of Kepler's laws of planetary motion and Newton's law of universal gravitation, which describe the motion of planets and other celestial bodies in our solar system. The corresponding terms for orbits around Earth are apogee and perigee for artificial satellites, and for the Moon's orbit as well.