What is the meaning of the advice, "show, don't tell"?

a. it can be more effective to let the reader figure out what a character is like, rather than telling the reader directly.
b. readers often prefer to be told directly what a character is like, in order to avoid ambiguity and confusion.
c. selecting precise adjectives to directly describe characters is the best way to the reader understand them.
d. most readers need to have characters explained to them directly, in addition to being shown specific examples.