Final answer:
Iago manipulates the situation when Bianca appears, showing jealousy over the handkerchief, to further incite Othello's jealousy and convince him of Desdemona's infidelity.
Step-by-step explanation:
After Bianca appears in Act 4, Scene 1, Iago begins a new part of his plot by manipulating Bianca's reaction to the handkerchief, which he had previously planted on Cassio. When Bianca storms in, jealous and upset about the handkerchief, Iago uses this situation to fuel Othello's jealousy even more.
Iago subtly suggests that Cassio and Desdemona are involved, and Bianca's display of jealousy is used by Iago to imply that Cassio is dismissive of Bianca because he favors Desdemona.