The correct option is D. chemical co-ordination. Explanation of the Correct Answer The mammalian endocrine system is a complex network of glands that produce and secrete hormones directly into the bloodstream. These hormones act as chemical messengers that regulate various physiological processes in the body, including growth, metabolism, reproduction, and mood. The term "chemical co-ordination" refers to the way these hormones coordinate and regulate bodily functions through chemical signals.

1. Hormonal Regulation: Hormones released by endocrine glands (such as the pituitary, thyroid, adrenal glands, and pancreas) travel through the bloodstream to target organs and tissues, where they bind to specific receptors. This binding triggers a response that helps maintain homeostasis, which is the body's stable internal environment.
2. Examples of Hormonal Functions:
3. Insulin: Produced by the pancreas, insulin regulates blood glucose levels.
4. Thyroid hormones: Regulate metabolism and energy levels.
5. Adrenaline: Released during stress, it prepares the body for a 'fight or flight' response.
6. Chemical Co-ordination vs. Other Systems: Unlike the nervous system, which uses electrical impulses for rapid communication, the endocrine system uses slower, longer-lasting chemical signals. This is crucial for processes that require sustained changes, such as growth and development.

Explanation of Why Other Options Are Incorrect A. Transmitting impulse: - This option refers to the function of the nervous system, not the endocrine system. The nervous system transmits electrical impulses through neurons to communicate quickly and efficiently. While both systems are involved in communication within the body, the endocrine system does not transmit impulses; it relies on hormones for signaling. B. Regulating body temperature: - While the endocrine system does play a role in regulating body temperature (for example, through the action of thyroid hormones), it is not its primary function. Body temperature regulation is primarily managed by the hypothalamus in conjunction with the autonomic nervous system, which can respond more rapidly to changes in temperature. C. Regulating osmotic pressure of blood: - Osmotic pressure regulation is indeed influenced by hormones (like aldosterone and antidiuretic hormone), but this is a specific function rather than a comprehensive description of the endocrine system's role. The endocrine system's primary function encompasses a broader range of chemical coordination beyond just osmotic pressure. Summary of Key Points

• The endocrine system is responsible for chemical co-ordination through hormone secretion.
• Hormones act as chemical messengers that regulate various bodily functions, including metabolism, growth, and mood.
• The endocrine system operates more slowly than the nervous system, which uses electrical impulses for rapid communication.
• While the endocrine system can influence body temperature and osmotic pressure, its primary role is broader and involves overall chemical regulation of bodily functions.

This understanding of the endocrine system is crucial for grasping how different physiological processes are interconnected and regulated in mammals.