Final answer:
Protection from foreign invasion is primarily provided by leukocytes, which are white blood cells involved in the immune response.
Step-by-step explanation:
Providing protection from foreign invasion is the job of white blood cells, also known as leukocytes. Leukocytes are part of the body's immune system and are crucial for defending the body against invading microorganisms, such as bacteria and viruses, and cells with mutated DNA. They are also responsible for cleaning up cellular debris. Erythrocytes, or red blood cells, have the job of delivering oxygen to the body's tissues and are not primarily involved in the immune response. Platelets, while essential for clotting and wound healing, do not provide protection from foreign invaders. Therefore, the answer to the question is A. leukocytes.