Galileo estimated the height of lunar mountains by: (a) measuring the height of clouds around the mountains (b) measuring the wavelength of light rays in his telescope (c) looking it up on the internet (d) observing illuminated mountain peaks in the dark portion of the moon (e) assuming lunar mountains are the same as earth mountains

Option (D) By observing illuminated mountain peaks in the dark portion of the moon.

Explanation:

Unlike the traditions at the time of Aristotle which considered that the celestial bodies are spherical and perfectly smooth bodies.

But based on Galileo's observations, the zone that was illuminated in the dark region of the moon near the line that separates the illuminated and the shadowy parts, known as terminator was noticed.

On the breaking of the dawn, as the sunlight hit the top of the lunar mountains it lightens up while the valleys are still dark.