An explosion that destroys the outer portion of a star is known as a
a. supernova.
b. supergiant.
c. protostar.
d. nebula

Technically all of these answers are incorrect , because when our Sun becomes a red giant and then collapses into a white dwarf, it will shed it's outer layers but this is not considered massive enough to be a supernova so technically none of your answers are correct but only one of your answers is actually an explosion. A would be the best answer

An explosion that destroys the outer portion of a star is known as a (C.) supernova.

Check out the Glencoe Earth Science textbook provided by PennFoster. The answer is on page 851. 

Answer were lower;

Just do the math, then think