The element carrying the entire weight of the plot is
a. stage directions
b. characters
c. structure
d. theme

I believe it is the characters that is carrying the entire weight of the plot.
Why?
because it is the character that relays the story's message to the audience or to the readers.
If the character were not able to do well on her acting, then possibilities are the audience were not able to get the message.

A: returning the ball before it bounces on the court