When Would Koch's Postulates Be Utilized

When would Koch's Postulates be utilized?

A) To determine the cause of a patient's illness in a hospital microbiology lab.
B) To develop a new antibiotic in a pharmaceutical lab.
C) To determine the cause of a new disease in a microbiology research lab.
D) To formulate a vaccine against a new pathogen in a genetic engineering lab.
E) Whenever the scientific method is used to investigate a microbiological problem.