The correct answer is D.
The decrease in white blood cell count in the human blood is known as leukopenia. This disease affects the body's ability to counter diseases and infections. A low blood count can result in the affected person becoming very susceptible to infections.
The primary function of white blood cells in the body is to fight and neutralize infections. Once the white blood cell count significantly dips below the normal level, the body's defense is compromised and so becomes susceptible to infections.