Final answer:
The mean orbital radius of astronomical objects orbiting the Sun is the semi-major axis, which is one-half the sum of the aphelion and perihelion. The correct answer is option D.
Step-by-step explanation:
The mean orbital radius of astronomical objects orbiting the Sun, for an elliptical orbit, is given by the semi-major axis of the ellipse.
The semi-major axis can be calculated as one-half the sum of the aphelion (ra) and perihelion (rp), which is mathematically expressed as a = ½(ra + rp).
This tells us that the correct formula to find the mean orbital radius in terms of aphelion and perihelion is a = (aphelion + perihelion)/2, thus the correct answer is (aphelion + perihelion)/2.