**Core Concept**
Mesophilic organisms are a type of microbe that thrive in moderate temperatures, typically between 20°C to 45°C. These bacteria and archaea are commonly found in a wide range of environments, including soil, water, and the human body.

**Why the Correct Answer is Right**
Mesophilic organisms have adapted to grow optimally in temperatures that are neither too hot nor too cold. This allows them to outcompete thermophilic organisms, which require higher temperatures, and psychrophilic organisms, which require lower temperatures. Mesophilic organisms often play a crucial role in decomposing organic matter and recycling nutrients in ecosystems.

**Why Each Wrong Option is Incorrect**
* **Option A:** Thermophilic organisms grow at temperatures above 50°C, which is much higher than the optimal range for mesophilic organisms.
* **Option B:** Psychrophilic organisms grow at temperatures below 15°C, which is much lower than the optimal range for mesophilic organisms.
* **Option C:** Obligate psychrophilic organisms require temperatures below 20°C to grow, whereas mesophilic organisms can thrive in a wider temperature range.

**Clinical Pearl / High-Yield Fact**
It's essential to note that mesophilic organisms can cause a range of infections in humans, including wound infections and septicemia, particularly in immunocompromised individuals.

**Correct Answer: D. 20-45°C**