Final answer:
The correct medium for Corynebacterium diphtheriae, which causes diphtheria and forms gray-black colonies, is Loeffler's medium.
Step-by-step explanation:
The question is inquiring about the type of agar that Corynebacterium diphtheriae would form gray-black colonies on. Corynebacterium diphtheriae, the bacterium that causes diphtheria, is an aerobe or facultative anaerobe and grows in palisades. It grows slowly and requires enriched media for culture.
Considering the options provided, Loeffler's medium is the correct answer. This medium is specifically designed for the isolation of Corynebacterium species and provides the necessary nutrients and conditions for the characteristic colony morphology of C. diphtheriae.