**Core Concept**
Nutrient agar is a general-purpose culture medium used to cultivate a wide variety of microorganisms. It is composed of agar, a polysaccharide derived from red algae, which provides a solid gel-like substrate for bacterial growth, and nutrients such as peptone, which serves as a source of amino acids and other essential nutrients.

**Why the Correct Answer is Right**
The correct concentration of agar in nutrient agar is 1.5% (15 g/L). This concentration allows for the growth of microorganisms while maintaining the structural integrity of the agar gel, which is essential for colony formation and observation under a microscope. The 1.5% concentration also prevents the agar from becoming too soft or too hard, thereby supporting optimal microbial growth.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because agar concentrations below 1.0% may result in a soft or liquid agar, which can lead to poor colony formation and difficulty in observing microorganisms.
**Option B:** This option is incorrect because agar concentrations above 2.0% may result in a hard or brittle agar, which can inhibit microbial growth and make it difficult to observe colonies.
**Option C:** This option is incorrect because agar concentrations around 1.0% may result in an agar that is too soft, leading to poor colony formation and difficulty in observing microorganisms.

**Clinical Pearl / High-Yield Fact**
It's essential to note that different types of agar may have varying concentrations of agar, and some may require specific concentrations for optimal growth of certain microorganisms. For example, MacConkey agar, used for the selective growth of gram-negative bacteria, requires a higher concentration of agar (2.0%) to inhibit the growth of gram-positive bacteria.

**Correct Answer: D. 1.5%**