The correct answer is:
An understanding of the number and size of leftover planetesimals in the early solar system.
The giant-impact hypothesis proposes that the Moon was created out of what was left of a collision between Earth and an astronomical body the size of Mars, around 4.5 billion years ago, in the Hadean eon. The heavy bombardment was a period when leftover planetesimals hit planets during the solar system's first hundred million years.
