Ans. is 'd' i.e., Tellurite medium - Best diagnosis of any bacteria is made by culture of specimen in the "selective media" and the selective medium for C. diphtheriae is tellurite agar. Loeffler's medium shows early growth (in 4-8 hrs), but it is not a selective medium for C. diphtheriae. Note : Most of the guides have given Loeffler's medium as correct answer. According to them diphtheria is an emergency, so we should make early diagnosis. Read text below : Green wood has not mentioned Loeffler's medium at all : "The recommended media include blood agar and a selective medium containing tellurite". Jawetz writes "Specific treatment must never be delayed for laboratory repos if the clinical picture is suggestive of diphtheria" - Although growth on Loeffler's medium helps in making an earlier diagnosis, as we have staed treatment on clinical ground, waiting for the growth on tellurite media would be the best option to diagnose the diphtheria as it acts as a selective media. So - For earliest diagnosis of diphtheria - Loeffler's serum slope - For best diagnosis of diphtheria - Tellurite medium