Answer:
b)Earth was struck by Mars-sized object; debris coalesced into the Moon
Explanation:
There was lots of theory which tells us the origin of moon.But in present time giant impact theory is accepted which states that moon formed due to collision of earth and one small planet and the size of that small planet was equal to the size of Mars.This Mars size planet called as Theia.
So the moon originated due to earth was struck by Mars-sized object; debris coalesced into the Moon.
Option b is correct.