The correct answer is - Sara suddenly became very ill; therefore, she went to see her doctor.
A semicolon is used to separate two sentences, two separate but also connected thoughts. It should come naturally as a pause within a sentence, which is why the first three options are incorrect as they break up the sentence in grammatically incorrect locations.
I'd say C, thats the only one that really makes sense with the pauses
Where are the questions to this ?
