Which is the best label for the dotted-line arrow

moon's motion without gravity

Explanation:

The reason it's moon's motion without gravity is because if you take away earths gravitational pull it would simply go in the direction it last was, and the last direction it was, was where the dotted arrow is.

Option (C)

Explanation: The dotted line in the picture depicts the direction in which the moon would move without the action of earth's gravitational pull and angular momentum. It is because of these two above conditions, the moon orbits around the earth, maintaining a circular path. The interaction of the moon and the earth also produces tidal effect.

Thus, the correct answer is option (C).