Answer: Othello thought that Cassio is talking about Desdemona.
Step-by-step explanation:
In Othello, Iago pretends to be Othello's friend, but plots against him. Iago is angry at him because Othello passed him over for a promotion with Cassio. Iago's plan is to make Othello think that Desdemona is unfaithful to him with Cassio, and thus destroy both his love life and his friendship with Cassio.
In Act IV, Scene I, Iago decides to put this plan into action. He reveals to the audience that he will talk to Cassio about his relationship with Bianca, so that Othello thinks that they are talking about Desdemona. He succeeds in this plan - Othello overhears their conversation and believes what he hears.