Final answer:
The gravitational acceleration is maximum at Earth's surface and approaches a minimum or zero at an infinite distance, where it becomes negligible. There is no altitude at which gravitational acceleration is constant, though it can be considered nearly constant over short altitude intervals.
Step-by-step explanation:
The altitude above Earth's surface at which the gravitational acceleration would be maximum is at the surface itself, because the gravitational force decreases with the square of the distance from the center of Earth, according to Newton's universal law of gravitation. The gravitational acceleration is minimum when the distance from the center of Earth is infinitely large, effectively reaching zero at an infinite distance. However, practically speaking, gravitational acceleration approaches zero far before reaching infinity at a point where Earth's gravity is negligible compared to other celestial bodies. The acceleration due to gravity does not stay constant with altitude; it decreases as one moves farther from the Earth's surface. Yet, locally, within a relatively small altitude range where the altitude change is minor, gravitational acceleration can be considered nearly constant.