The correct option is A the diagnosis of the diseases by analysing the chemistry of the body fluid of the affected organisms.
Pathology is the study of the cause of a disease. This is done by testing the body fluids for the presence of microorganisms or chemicals indicative of the disease.
Treating diseases with drugs or chemicals comes under the purview of medicine.
Study of the functions of the body systems of animals and plants, covering vital activities at cellular, tissue and organ level is called physiology.
Study of structural organization of living beings at tissue level is called histology.