**Core Concept**
Neisseria gonorrhoeae and Neisseria meningitidis are two closely related species of Gram-negative diplococci that can cause different types of infections. The primary goal of differentiation is to identify the causative agent and direct targeted therapy, as both species have different antibiotic sensitivities.

**Why the Correct Answer is Right**
The correct answer is based on the fact that Neisseria gonorrhoeae and Neisseria meningitidis can be differentiated by their ability to grow at different temperatures. Neisseria gonorrhoeae is unable to grow at 40°C, whereas Neisseria meningitidis is able to grow at this temperature. This difference in temperature tolerance is due to the presence of a specific enzyme, alkaline phosphatase, in Neisseria meningitidis that allows it to grow at higher temperatures.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the differentiation of Neisseria gonorrhoeae and Neisseria meningitidis.
* **Option B:** While both species are susceptible to penicillin, their antibiotic sensitivities differ, and this option does not address the temperature difference.
* **Option C:** This option is incorrect because it does not accurately describe the method of differentiation.

**Clinical Pearl / High-Yield Fact**
It's essential to note that both Neisseria gonorrhoeae and Neisseria meningitidis can cause severe infections, and prompt identification and treatment are crucial to prevent complications.

**Correct Answer: D. Temperature tolerance.