White blood cells are the part of blood that carry out the function of body defense.

1. Red blood cells (option 1) primarily carry oxygen to the cells and remove carbon dioxide. They are not involved in the body`s defense against pathogens or foreign substances.

2. White blood cells (option 2) are responsible for protecting the body against infections and diseases. They are the main players in the immune system and help to identify and destroy foreign invaders, such as bacteria, viruses, and parasites. There are different types of white blood cells, each with specific roles in the defense mechanism.

3. Platelets (option 3) are responsible for blood clotting. When there is an injury and blood vessels are damaged, platelets help to form a clot to stop bleeding. While they do contribute to the overall functioning of the immune system, they are not the primary defense mechanism against pathogens.

4. Hemoglobin (option 4) is a protein found in red blood cells that helps carry oxygen to the cells. It does not have a direct role in body defense.

In conclusion, white blood cells are the specific part of the blood that carries out the function of body defense.