Question

Which sentence demonstrates correct use of the semicolon?

Answer 1

Choice D is the correct answer.

Why?
Basically, just try to finish the sentence by asking questions:
A) Everyone needs (what?)
B) We need to pick up (whom?)
C) My favorite British playwright is (who?)
D) Sara suddenly became very ill (there is no information left out, no question to fill the sentence, thus use the ';').

Answer 2

D, Sara suddenly became very ill; therefore, she went to see her doctor.