The correct answer is option 2: White blood cells.

White blood cells, also known as leukocytes, are a crucial part of the body`s defense system. They play a vital role in protecting the body against infections and diseases. Unlike red blood cells, which are responsible for carrying oxygen, white blood cells are primarily involved in the immune response.

White blood cells are produced in the bone marrow and can be found both in the bloodstream and various tissues in the body. They have the ability to move out of blood vessels and into tissues to target and eliminate harmful pathogens, such as bacteria, viruses, and fungi.

There are different types of white blood cells, each with its specific functions. For example, neutrophils are involved in the initial response to infection, while lymphocytes help produce antibodies and coordinate the immune response. Monocytes are responsible for engulfing and destroying dead cells and debris.

In summary, white blood cells are the part of the blood that carries out the function of body defense by identifying and neutralizing harmful pathogens.