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 cannot be used to investigate a microbiological problem