This system has three primary functions, first it returns excess interstitial fluid to the blood, second is for the absorption of fats and fat-soluble vitamins from the digestive system and the subsequent transport of these substances to the venous circulation and the third and probably most well-known function of this system is defense against invading microorganisms and disease. A. Lymphatic System B. Digestive System C. Nervous System D. Cardiovascular System

The correct answer is **A. Lymphatic System**.<br /><br />Here's why:<br /><br />* **Returning excess interstitial fluid to the blood:** The lymphatic system collects fluid that leaks from blood capillaries and returns it to the bloodstream, preventing swelling and maintaining fluid balance.<br />* **Absorption of fats and fat-soluble vitamins:** Specialized lymphatic vessels called lacteals absorb fats and fat-soluble vitamins in the small intestine and transport them to the circulatory system.<br />* **Defense against invading microorganisms:** The lymphatic system contains lymph nodes and other lymphoid tissues that house immune cells (like lymphocytes) which fight infections and diseases.<br /><br /><br />The other options are incorrect:<br /><br />* **Digestive System:** While involved in absorbing nutrients, the digestive system doesn't return interstitial fluid to the blood or directly participate in immune defense in the same way the lymphatic system does.<br />* **Nervous System:** The nervous system controls and coordinates bodily functions but isn't directly involved in fluid balance, fat absorption, or immune defense in the way described.<br />* **Cardiovascular System:** The cardiovascular system transports blood, but the return of interstitial fluid and the specific immune functions described are primarily the roles of the lymphatic system.<br />