Which of the following is a symptom of depression?

Losing interest in things you once enjoyed is a well-recognized symptom of depression, characteristic of what is often referred to as anhedonia. This symptom reflects a significant change in an individual's emotional response and engagement with activities that previously brought joy or satisfaction. It is a common manifestation of depression and can significantly affect a person's quality of life, social interactions, and overall emotional health.

Other options describe feelings or states that are typically contrary to symptoms associated with depression. For instance, excessive energy and feeling overly confident generally align more with states of increased mood or levels of activity, which are not consistent with depressive symptoms. Constant headaches, while they can occur due to various reasons including stress or anxiety, are not specifically indicative of depression in the way that the loss of interest in activities is. Thus, the choice reflecting a loss of interest is the most aligned with core depressive symptoms.