Selye a. failed to acknowledge the role of the environment in adapting to stress. b. conducted extensive research on the effects of stress on the immune system. c. focused on the cognitive component of the stress response. d. recognized that both happy and sad life events can be sources of stress.

The correct answer is "letter d. recognizes, wherein both happy and sad life-events can be causes and sources of stress."

janos Hugo Bruno Selye or commonly referred to as Hans Selye was a known endocrinologist, who conducted a lot of scientific works that is related to stress and the responses of organisms to various stressors.