4. Why is the work of Elisabeth Kủbler-Ross important? Her work showed how patients with incurable illnesses react emotionally. Her work began to explain life-expectancy variations based on race and sex. Her work helped define the criteria used to establish clinical death. Her work opened up a brand-new area of psychological research.

Elisabeth Kübler-Ross is best known for her work on the emotional responses of patients with incurable illnesses. Her book "On Death and Dying" introduced the concept of the five stages of grief, which has had a significant impact on the understanding and treatment of grief and terminal illness. Therefore, the correct answer is:<br /><br />Her work showed how patients with incurable illnesses react emotionally.