Which of the following correctly uses a semicolon?

after i bought the book; i started to read right away.

the characters were; in my opinion; very realistic.

this is the third book in the series; which i really love.

the author has written four other books; this is book five.

The sentence that uses a semicolon correctly is the last choice: The author has written four other books; this is book five.  The purpose of a semicolon is to first of all separate two different sentences.  Its secondary purpose is to come in the place of a conjunction preceded by a comma and it can also take the place of a period.  Hope that this answer helps you!

The correct answer for the first question would be: The author has written four other books; this is book five. (the other sentences could be corrected by simply using commas) And the correct answer for the second question is: probably: Most people eat cereal, it (im sure there was more to the sentence, but even if there wasn't, it didn't end with a period, therefore making it a run-on)