Final answer:
Helena hopes to gain Demetrius' favor by telling him about Lysander and Hermia's escape plan.
Step-by-step explanation:
By telling Demetrius of Lysander and Hermia's flight, Helena hopes to gain his favor and potentially win his love. Helena is deeply in love with Demetrius, but he is in love with Hermia. Helena believes that by disclosing Lysander and Hermia's plan to elope, she will be seen as loyal and trustworthy by Demetrius, which may make him reconsider his feelings for her.