I think the answer is C. An element or compound that cannot be physically separated. Sorry if im wrong.
Answer:
C. the particles of a gas collide with each other and with other objects
It is called a chemical change
For the answer to the question above,
If you mean planet, that would be Mercury. There are other objects in our solar system that could fit the criteria.
Our moon, Ganymede, and Callisto are about a quarter the size of Earth and have no atmosphere or moon.