What is the difference between a crater and a caldera?
a. craters are only formed from asteroid impacts.
b. calderas are only formed from asteroid impacts.
c. craters are formed by ejecting material during volcanic explosions while calderas are formed from the inward collapse of volcano tops during explosive eruptions.
d. calderas are formed by ejecting material during volcanic explosions while craters are formed from the inward collapse of volcano tops during explosive eruptions.

C) Craters are formed by ejecting material during volcanic explosions while calderas are formed from inward collapse of volcano tops during explosive eruptions.

Calderas form when the volcanic eruption or the eruption of lava creates a huge empty magmatic chamber below the surface of the earth which later form a depression due to collapsing back into the chamber.  

Calderas are not visible with a naked eye; they can only be seen on images taken from out of space. An example of the gigantic caldera is Yellowstone National Park. Craters, however, results from volcanic eruptions which cause vents were magma and other material escapes to form a cone shape. An example is Mt. St Helen.