Answer: Emilia gives Iago Desdemona's handkerchief, which he later plants in Cassio's room in order to make Desdemona seem unfaithful.
Step-by-step explanation:
Iago, passed over for a promotion by Othello, deeply hates Othello and pretends to be his friend with an ultimate goal to destroy him.
Iago's wife, Emilia, plays a crucial role in this plot. At first, it seems that Emilia is rather passive character. She is naive and just listens to her husband whatever the cost. When she finds Desdemona's handkerchief, she states that, on multiple occasions, her husband asked her to steal it for him. She is happy to finally bring it to him. Iago plants a handkerchief in Cassio's room, as a proof for Othello that his wife has been unfaithful.
As the story progresses, Emilia becomes suspicious, and when Othello kills Desdemona for her infidelity, Emilia reveals to him that the handkerchief was stolen. In this way, Emilia exposes Iago and his true intentions, and Iago kills her.