The correct option is C Saprophytic bacteria
The antibiotics are substances produced by the natural metabolic processes of spore forming aerobic bacteria and fungi as their natural defence against other microorganisms. Immune host is the one which has acquired immunity against certain antigen by administration of antibiotics. It produces antibodies, not the antibiotics which makes option A incorrect. Commercial production of antibiotics by parasitic fungi needs their culture in live host cells which is a tedious and expensive process; Option B is incorrect. Saprophytic bacteria feed on dead and decaying organic matter and therefore, can easily be cultured on synthetic medium; option C is correct. Since, viruses are metabolically inactive outside the host cell and depend on host for their own nutritional requirements, they do not produce antibodies. Option D is incorrect. Thus, the correct answer is option C.