Impacts on the terrestrial planets and the Moon a. are more common than they used to be. b. have occurred at approximately the same rate since the Solar System formed. c. are less common than they used to be. d. periodically become more common and then are less common for a while. e. never occur anymore.

The formation of our Solar System began about 4.6 billion years ago from a giant cloud of gas and dust. This cloud, known as the solar nebula, collapsed under its own gravity, causing it to spin and flatten into a disc. The Sun formed at the center, while the remaining material clumped together to form the planets, including the terrestrial ones: Mercury, Venus, Earth, and Mars.
This early period was highly turbulent, characterized by frequent and violent collisions as planetesimals (small bodies) merged to form the larger planets. These collisions were essential for building the planets we know today but also made the early Solar System a hazardous place.
During this stage, impacts were frequent due to the chaotic environment and abundance of debris. It was common for large objects to collide, contributing to the growth and evolution of planets.
Understanding this helps explain why the impact frequency has changed over time.

In the beginning, the Solar System was filled with leftover debris from the planet formation process. This debris consisted of planetesimals, small fragments, and dust.
This material was constantly moving and colliding, leading to a high frequency of impacts on the newly formed planets and moons. These impacts played a role in shaping the surfaces and geological features we observe today.
As the Solar System aged, much of this debris either became part of larger bodies like planets and moons or was ejected into space by gravitational interactions. This process significantly reduced the number of potential impactors over time.
For example, the Late Heavy Bombardment was a period around 4 billion years ago when the inner planets and the Moon experienced a spike in impact events, leaving many craters that we can still see on the Moon's surface.

Today, the impact frequency on terrestrial planets and the Moon is much lower compared to the early Solar System. Most of the early debris has been cleared out, either by being accreted into planets or expelled from the Solar System.
However, impacts still occur. They are just less common. Most of the impacting bodies today are remnants from the early Solar System, such as asteroids and comets, but their numbers are far fewer.
By comparing the options from the exercise, we can deduce that statement 'c. are less common than they used to be' is correct. This is because the early chaotic environment had more frequent impacts, while today's Solar System is relatively stable with fewer objects left that can cause significant impacts.