Among the options given, D) MacConkey agar is the differential media.
MacConkey agar is specifically designed to differentiate between lactose fermenters and non-fermenters. It contains lactose and a pH indicator that causes color changes based on the fermentation process. When bacteria that can ferment lactose grow on this medium, they produce acid, leading to a change in color (typically pink) of the colonies. This characteristic makes MacConkey agar a useful tool in microbiology for isolating and identifying enteric gram-negative bacteria.
In contrast, blood agar is an enriched medium that supports the growth of a wide range of organisms and allows for the observation of hemolytic activity but is not strictly differential in the way MacConkey agar is. Koser citrate medium is a selective medium used for the differentiation of organisms that can utilize citrate as their sole carbon source, and nutrient broth is a general-purpose medium that lacks differential properties.