Final answer:
Complement proteins are part of the immune system and can be found in the blood, lymph, and extracellular fluid to help destroy extracellular pathogens.
Step-by-step explanation:
Complement proteins can be found in the blood, lymph, and extracellular fluid. These complement proteins are a part of the complement system, which consists of around 20 soluble proteins synthesized by liver cells and macrophages. Their primary function is to attack and destroy extracellular pathogens, playing a significant role in both the innate and adaptive immune responses. Therefore, the correct answer to the question of where complement proteins can be found is: e. A, B & C.