The correct option is B. Homeostasis. Explanation of the Correct Answer Homeostasis is the process by which living organisms regulate their internal environment to maintain a stable, constant condition, despite changes in the external environment. This is crucial for the survival of organisms, as it allows them to function optimally even when faced with varying external conditions such as temperature, pH, and the concentration of ions and nutrients.

1. Definition of Homeostasis: Homeostasis involves various physiological processes that work together to keep the internal environment of the body stable. For example, humans maintain a body temperature around 37°C (98.6°F) through mechanisms such as sweating to cool down or shivering to generate heat.
2. Mechanisms of Homeostasis: The body uses feedback systems to maintain homeostasis. These systems typically involve:
3. Sensors: Detect changes in the internal environment (e.g., temperature receptors in the skin).
4. Control Center: Processes the information and determines the appropriate response (e.g., the hypothalamus in the brain).
5. Effectors: Carry out the response to restore balance (e.g., sweat glands or muscles).
6. Examples of Homeostasis:
7. Temperature Regulation: When the body temperature rises, sweat glands produce sweat, which evaporates and cools the body. Conversely, when it drops, the body may shiver to generate heat.
8. Blood Glucose Levels: The pancreas releases insulin to lower blood sugar levels after eating, and glucagon to raise them when fasting.

Explanation of Why Other Options Are Incorrect A. Metabolism: - Metabolism refers to all the chemical reactions that occur within an organism to maintain life, including those that break down nutrients for energy (catabolism) and those that build up cellular components (anabolism). While metabolism is essential for providing the energy needed for homeostasis, it does not specifically describe the process of maintaining a stable internal environment. C. Cellular Respiration: - Cellular respiration is a specific metabolic process that converts glucose and oxygen into energy (ATP), carbon dioxide, and water. It is a part of metabolism and is crucial for energy production, but it does not encompass the broader concept of maintaining internal stability in response to external changes. D. Photosynthesis: - Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. This process is vital for producing food and oxygen but is not related to the internal regulation of an organism's environment. Summary of Key Points

• Homeostasis is the process of maintaining a stable internal environment despite external changes.
• It involves feedback systems with sensors, control centers, and effectors to regulate conditions like temperature and blood glucose levels.
• Other options (metabolism, cellular respiration, photosynthesis) are related to energy production and biochemical processes but do not specifically address the concept of internal stability.

Revision Summary

• Homeostasis is crucial for survival, allowing organisms to adapt to external changes.
• It involves sensors, control centers, and effectors working together.
• Key examples include temperature regulation and blood glucose control.
• Metabolism, cellular respiration, and photosynthesis are important but do not define homeostasis.