Given that the Earth was struck by a Mars-sized body about 4.5 billion years ago. Let's determine the result of the collision between the Earth and the body.
According to the giant-impact hypothesis which is also called the Big Splash, the Moon was formed when there was a collision between the Earth and a Mars-sized planet.
This collision is believed to have happened about 4.5 billion years ago.
Also, the name of this Mars-sized object which collided with the Earth is said to be Theia.
Therefore, the result of the collision between the Earth and the Mars-sized body which occurred about 4.5 billion years was the formation of the moon.
ANSWER:
A. The formation of the moon.