What is one major difference between a monologue and a soliloquy?
a. monologues are delivered by an actor alone onstage.
b. monologues are not meant to be heard by other characters.
c. soliloquys are not meant to be heard by other characters.
d. soliloquys are longer and more serious than monologues.

C is correct purely by process of elimination. First you have to know what the definitions of both words or have an example in mind. Usually monologues are just long or serious dialogue spoken by one character. Other characters can hear and can respond. A soliloquy is a theatric aspect. This strategy lets the audience have an understanding about what the actor is thinking. They aren't usually heard by other characters and are not meant to be.