The right option is; d. Immune system
The immune system is responsible for defending the body against harmful substances.
The immune system is the body’s defense system that protects the body against infectious pathogens and other invaders. The immune system is composed of many biological structures (special cells, proteins, tissues, and organs) and processes that functions together to protect the body. White blood cells are one of the important cells of the immune system that destroy harmful substances or disease-causing organisms that invade body systems.