C. revealing Lagos plan for taking revenge on Othello.
In this passage, Iago is formulating a plan to ruin Cassio's reputation with Othello by suggesting that Cassio is having an affair with Desdemona. The passage reveals Iago's intention to use Cassio to achieve his own goals and to take revenge on Othello for promoting Cassio instead of him. Therefore, the main purpose of this passage is to reveal Iago's plan for taking revenge on Othello.