I believe the answer you are looking for would be the second answer choice, "We read Shakespeare's tragedy, Othello, after reading two of his comedies."
This is because when information already speaking of something mentioned, is referred to to clarify a subject, it must be separated by commas. (that's how I remember)
ex: My mother, the queen, is placing a new law down to make everyone have equal rights.
ex: Today I went to Ms. Green's, the principal's, office.