Concept:The underlined expression means that Roderigo will benefit personally if he makes Othello's wife unfaithful.
Explanation:In this extract, Iago is encouraging Roderigo to take revenge on Othello.
"Cuckold him" means to make a husband unfaithful to his wife? Wait exactly means to make the husband a cuckold by making his wife unfaithful to him.
Here the husband is Othello, and the wife is Desdemona.
Iago says "thou dost thyself a pleasure," meaning Roderigo will gain personal satisfaction.
Iago adds "me a sport," meaning it will entertain Iago as well.
So the underlined expression shows that Roderigo will gain pleasure if he succeeds in making Desdemona cheat on Othello.
Answer:B. you gain by making his wife cheat on him