White blood cells, also known as leukocytes, play a vital role in the immune system and are responsible for protecting the body against diseases. This makes option-4 "protect body against diseases" the correct answer.

White blood cells defend the body by identifying and destroying any foreign substances such as viruses, bacteria, and other pathogens that may cause infections or diseases. They do this by either engulfing the pathogens or releasing chemicals to neutralize them. White blood cells are also involved in the process of inflammation, which is a natural response to infection or injury.

Options 1, 2, and 3 are incorrect because white blood cells are not responsible for carrying oxygen (that`s the role of red blood cells), assisting in clot formation (that`s primarily done by platelets), or producing more red cells (that`s the function of the bone marrow).

Thus, option-4 is the most accurate choice for the main function of white blood cells in the body.