The answer is C: because growth at 37°C would be ideal for revealing bacteria that are human pathogens. 37°C is equivalent to 98.6°F, the normal body temperature for humans. If bacteria are reproducing at this temperature in a petri dish, they are also most likely reproducing in the body.
Cultures are made so doctors can be sure a person is sick with a specific bacteria often in order to make sure they are taking the right medication to get better. Choice A doesn't make sense, because we wouldn't want to kill the bacteria we are trying to study. Bacteria that makes us sick is harmful bacteria and is what we are trying to isolate. Choice B doesn't make sense, because they are only being incubated at one temperature, not a range or variety. Choice D is harder to rule out, but again the doctor wants the bacteria to reproduce so they can be sure that's what is causing the infection, so it wouldn't make sense that we would put the bacteria in a temperature they would not reproduce.
