Final answer:
MacConkey agar plates are a type of microbiological medium that is both selective for gram-negative bacteria and differential for lactose fermenters. They contain bile salts and crystal violet to inhibit gram-positive bacteria and a pH indicator to identify lactose-fermenting bacteria. Thus, the option D is the correct answer.
Step-by-step explanation:
The correct answer to the question is D) both a differential and a selective medium. MacConkey agar plates are a type of microbiological growth medium that serve both selective and differential purposes. These plates contain bile salts and crystal violet, which inhibit the growth of most gram-positive bacteria, thus being selective for gram-negative bacteria. Additionally, MacConkey agar contains neutral red, a pH indicator that turns pink at low pH levels. This feature enables it to act as a differential medium by allowing for the identification of lactose-fermenting bacteria. Lactose fermenters produce acid which lowers the pH, causing the neutral red to turn pink, thereby visually differentiating them from non-lactose fermenting colonies.