**Core Concept**
Selective media are growth media used in microbiology to promote the growth of specific microorganisms while inhibiting the growth of others. These media contain ingredients that selectively allow certain microbes to grow while suppressing the growth of others, often through the inhibition of enzymes or the utilization of specific nutrients.

**Why the Correct Answer is Right**
The correct answer is an example of a selective medium that contains an ingredient that inhibits the growth of unwanted microorganisms. For instance, if the correct answer is a medium containing an antibiotic, it would inhibit the growth of bacteria while allowing other microbes to grow. The selective medium helps in isolating and culturing specific microorganisms from a mixed culture.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not contain any information about a selective medium.
**Option B:** This option is incorrect because it is a type of growth medium that is used to promote the growth of a wide range of microorganisms, not a selective medium.
**Option C:** This option is incorrect because it is a type of enrichment medium that is used to promote the growth of specific microorganisms by providing them with a specific nutrient, but it is not a selective medium.

**Clinical Pearl / High-Yield Fact**
To remember the difference between selective and enrichment media, you can use the following mnemonic: "Selective media are like a filter, allowing certain microbes to grow while keeping others out, while enrichment media are like a buffet, providing specific nutrients to promote the growth of desired microbes."

**Correct Answer: C. MacConkey agar**