What body system works to keep internal conditions stable?

The correct choice is the autonomic nervous system, which plays a critical role in maintaining homeostasis, or the stability of internal conditions within the body. This system operates largely unconsciously and regulates bodily functions such as heart rate, respiration rate, digestion, and glandular activity. By monitoring and responding to various physiological changes, the autonomic nervous system ensures that conditions remain within a narrow range that is optimal for bodily functions.

For example, when the body experiences stress, the autonomic nervous system can activate the "fight or flight" response, increasing heart rate and blood pressure to enhance immediate physical performance. After the stressor is removed, it helps to return the body to a state of rest by lowering these functions.

The immune system, while crucial for defending against pathogens, does not regulate internal stable conditions; instead, it reacts to external threats. The endocrine system also plays a vital role in maintaining homeostasis through hormonal regulation, but its influence is more gradual compared to the immediate actions of the autonomic nervous system. The circulatory system primarily facilitates the transportation of nutrients and oxygen throughout the body and does not actively maintain internal conditions on its own.