**Core Concept**
Bacteria that grow optimally between 25-40°C are called mesophiles. Mesophiles are a group of microorganisms that thrive in moderate temperatures, typically between 20-45°C. This temperature range is ideal for the metabolic processes of these bacteria.

**Why the Correct Answer is Right**
Mesophiles, including species like Escherichia coli, Staphylococcus aureus, and Bacillus subtilis, have enzymes that maintain optimal activity within this temperature range. The cell membranes of mesophiles are composed of phospholipids with acyl chains that are fluid and mobile at moderate temperatures, allowing for efficient membrane transport and metabolic processes. This is in contrast to thermophilic bacteria, which have adapted to grow at higher temperatures, and psychrophilic bacteria, which grow best at lower temperatures.

**Why Each Wrong Option is Incorrect**
**Option A:** Psychrophiles grow optimally at temperatures below 15°C, which is not the case for the bacteria described in the question.
**Option B:** Thermophiles grow optimally at temperatures above 45°C, which is not the temperature range specified in the question.
**Option C:** Obligate pathogens are bacteria that require a specific host or environment to grow, and temperature is not the sole factor determining their growth.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the optimal growth temperature of bacteria can impact their virulence and pathogenicity. Understanding the temperature preferences of bacteria can help in predicting their ability to cause disease in humans.

**Correct Answer:** C.