If the chemical X-gal is added to nutrient agar, lactose fermenting colonies turn blue, while non-fermenting colonies appear white. What sort of medium is X-gal agar?

C. Differential.

Explanation:

Differential media are bacteriological growth media that contain specific ingredients to allow one to distinguish selected species or categories of bacteria by visual observation. Examples of differential media includes:

Blood agar (used in strep tests) contains bovine heart blood that becomes transparent in the presence of β-hemolytic organisms such as Streptococcus pyogenes and Staphylococcus aureus.

Eosin methylene blue is differential for lactose fermentation.

Streptococcuseosin methylene blue (EMB), which is differential for lactose and sucrose fermentation.