Which fluid circulates in the lymphatic system and helps remove waste and impurities?

Lymph is the correct answer because it is a vital fluid within the lymphatic system, responsible for transporting waste products, impurities, and excess fluids away from tissues. The primary function of lymph is to filter these substances through the lymph nodes, where immune responses can also be activated to help fight infections. This process is essential for maintaining the body's fluid balance and ensuring the immune system can effectively respond to threats.

Plasma, while also a fluid found in the circulatory system, primarily facilitates the transport of nutrients, hormones, and waste in the blood, rather than functioning specifically in the lymphatic system. Serum is the liquid that remains after blood has clotted and contains various proteins, but it does not play a direct role in the removal of waste through the lymphatic channels. Interstitial fluid surrounds tissue cells and provides a medium for nutrient and waste exchange at the cellular level but is distinct from lymph, which is collected from this interstitial fluid and transported through the lymphatic vessels.