**Core Concept**
Exercise is prescribed as an adjuvant treatment for depression, primarily due to its effects on the brain's neurotransmitter systems and neuroplasticity. This concept involves the interaction between physical activity, mood regulation, and the central nervous system.

**Why the Correct Answer is Right**
Exercise stimulates the release of various neurotransmitters, including norepinephrine, serotonin, and endorphins. These neurotransmitters play a crucial role in modulating mood, motivation, and emotional regulation. The increased production of these neurotransmitters contributes to improved mood and reduced symptoms of depression. Additionally, exercise promotes neuroplasticity by encouraging the growth of new neurons and forming new neural connections in areas of the brain involved in mood regulation. This process is often referred to as "neurogenesis."

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because, while stress reduction is a potential benefit of exercise, it is not the primary mechanism by which exercise acts as an adjuvant treatment for depression.
**Option B:** This option is incorrect because, although sleep disturbances are common in individuals with depression, exercise does not primarily act by improving sleep quality in the context of depression treatment.
**Option C:** This option is incorrect because, while exercise can lead to weight loss, this is not the primary mechanism by which it acts as an adjuvant treatment for depression.

**Clinical Pearl / High-Yield Fact**
A mnemonic to remember the key neurotransmitters involved in exercise's antidepressant effects is "NESE" (Norepinephrine, Endorphins, Serotonin, and Energy). This acronym can help students recall the essential components of exercise's mechanism in treating depression.

**Correct Answer:**
C.