**Core Concept**
Blood agar is a type of enriched medium used in microbiology for the isolation and cultivation of various microorganisms, particularly those that require additional nutrients for growth. This medium is composed of a nutrient agar base supplemented with sheep or rabbit blood, which provides essential growth factors and nutrients for microorganisms.

**Why the Correct Answer is Right**
The addition of blood to the agar base provides a rich source of nutrients, including hemin, which is essential for the growth of certain microorganisms, such as Haemophilus influenzae. The blood also contains a source of iron, which is necessary for the growth of some bacteria. The blood agar medium is particularly useful for the isolation of alpha-hemolytic streptococci, which produce partial hemolysis of red blood cells, resulting in a greenish discoloration of the agar.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because blood agar is not a selective medium, which means it does not inhibit the growth of certain microorganisms while allowing others to grow. Blood agar is a non-selective medium that allows a wide range of microorganisms to grow.

**Option B:** This option is incorrect because blood agar is not a differential medium, which means it does not distinguish between different types of microorganisms based on their metabolic activities. Blood agar is a non-differential medium that does not provide any specific characteristics to distinguish between different microorganisms.

**Option C:** This option is incorrect because blood agar is not a synthetic medium, which means it is not composed of artificial nutrients. Blood agar is a semi-synthetic medium that uses a nutrient agar base supplemented with sheep or rabbit blood.

**Clinical Pearl / High-Yield Fact**
Blood agar is a versatile medium that can be used to isolate a wide range of microorganisms, including bacteria, fungi, and viruses. However, it is essential to note that blood agar is not suitable for the isolation of all microorganisms, and other specialized media may be required for specific types of microorganisms.

**Correct Answer: C. Blood agar is a semi-synthetic medium used in microbiology for the isolation and cultivation of various microorganisms.