The correct answer is option 'D', which is Neutrophils. Neutrophils are a type of white blood cell (WBC) and they are the most abundant type of WBC in the body. They play a critical role in the immune response against bacterial and fungal infections.

Neutrophils are produced in the bone marrow and are released into the bloodstream where they circulate and migrate to sites of infection or inflammation. They are highly mobile cells and can quickly move to the site of injury or infection.

Here is a detailed explanation of why neutrophils are the maximum number of WBCs in the body:

1. Function: Neutrophils are phagocytic cells, which means they can engulf and digest microorganisms, such as bacteria and fungi. They release toxic substances and antimicrobial proteins to destroy pathogens. Neutrophils are the first cells to arrive at the site of infection or injury and play a crucial role in the early stages of the immune response.

2. Lifespan: Neutrophils have a short lifespan of only a few hours to a few days. This is because they are highly active and expend their energy quickly. They undergo programmed cell death, called apoptosis, after fulfilling their function. As a result, the bone marrow continuously produces new neutrophils to maintain their high numbers in the body.

3. Production: Neutrophils are constantly produced in the bone marrow in a process called granulopoiesis. This process is regulated by various growth factors and cytokines. Neutrophil production increases during infections or inflammation to meet the increased demand.

4. Circulation: Neutrophils are released into the bloodstream from the bone marrow, where they circulate until they are needed. They constantly move between the bloodstream and tissues, patrolling for any signs of infection or injury.

5. Migration: When there is an infection or injury, neutrophils are attracted to the site by chemical signals released by damaged tissues or by other immune cells. They migrate out of the bloodstream and into the infected tissues using a process called chemotaxis. This ability to migrate to the site of infection is critical for their function.

Due to their high numbers, rapid production, and ability to migrate to sites of infection, neutrophils are considered the first line of defense against infections. Their abundance in the body makes them the maximum number of WBCs.