Corynebacterium diphtheriae can be grown within 6-8 hours on:: (A) Potassium tellurite media with iron, (B) McConkey’s agar, (C) Dorset egg medium, (D) Loeffler’s serum slope
Answer is D. On Loeffler’s serum slope, C. diphtheriae grows after 4–6 hours at 37°C producing small, circular, and white opaque colonies. 
If no colonies are seen, the medium is reincubated further for 24 hours.

Parija SC. Textbook of Microbiology & Immunology. Elsevier Health Sciences; 2014. Page:218