Which of the following colony appearances on Levine's EMB agar indicates lactose fermentation?

The correct choice is associated with the production of acid and possibly gas due to the fermentation of lactose by certain bacteria. On Levine's Eosin Methylene Blue (EMB) agar, colonies that ferment lactose create acidic byproducts that lower the pH of the medium. This change in pH causes the dyes eosin and methylene blue to precipitate, leading to the formation of black or purple colonies.

These characteristics are particularly indicative of coliforms, such as Escherichia coli, which ferment lactose vigorously, often resulting in dark, shiny colonies with a green metallic sheen. Thus, the appearance of black or purple colonies signifies that lactose fermentation has occurred on EMB agar.

In contrast, other appearances like brick red, colorless, and white suggest that lactose fermentation did not take place, which is why they do not indicate successful lactose utilization.