Final answer:
The immune system, consisting of the innate and the adaptive immune systems, is primarily responsible for protection against foreign invaders.
Step-by-step explanation:
The body system primarily responsible for protection from foreign invaders is the D) Immune system.
The immune system is a complex network that defends the body against external threats like pathogens and internal threats such as cancerous cells. There are two major subsystems within the immune system: the innate immune system and the adaptive immune system. The innate immune system provides a non-specific defense against pathogens, acting as the first and second lines of defense through features like physical barriers (skin, mucus) and chemical responses that include inflammation and fever. The adaptive immune system, on the other hand, provides a highly specific defense against pathogens. It develops a memory of past infections, allowing for a stronger and quicker response if the same pathogen invades again in the future.