What is ironic about Cassio and Iago gave their respective reputations?

a. Cassio has fairly gained his reputation while Iago has fairly lost his reputation.
b. Cassio has fairly lost his reputation while Iago has fairly gained his reputation.
c. Cassio has unfairly gained his reputation while Iago has unfairly lost his reputation.
d. Cassio has unfairly lost his reputation while Iago has unfairly gained his reputation