**Core Concept**
The cultivation of anaerobic organisms requires specific conditions to prevent oxygen from interfering with their growth. Anaerobic bacteria thrive in environments devoid of oxygen, using alternative metabolic pathways to generate energy.

**Why the Correct Answer is Right**
The correct answer is related to the cultivation techniques used for anaerobic organisms. Anaerobic bacteria are typically grown in an anaerobic chamber or a sealed tube containing a reducing agent, such as hydrogen gas or carbon dioxide, to maintain an oxygen-free environment. The use of a CO2-rich environment is essential for the growth of some anaerobic bacteria, as it provides the necessary carbon dioxide for their metabolic processes. This is particularly true for bacteria that use the Wood-Ljungdahl pathway, which involves the reduction of carbon dioxide to produce energy.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe a method for cultivating anaerobic organisms. While agar can be used as a growth medium, it is not sufficient on its own to support anaerobic growth.
**Option B:** This option is incorrect because it implies that anaerobic organisms can be grown in the presence of oxygen. However, anaerobic bacteria are highly sensitive to oxygen and will not grow in environments containing even small amounts of oxygen.
**Option C:** This option is incorrect because it is not a specific technique for cultivating anaerobic organisms. While centrifugation can be used to separate cells from a growth medium, it is not a method for creating an anaerobic environment.

**Clinical Pearl / High-Yield Fact**
When cultivating anaerobic bacteria, it's essential to use a reducing agent, such as hydrogen gas or carbon dioxide, to maintain an oxygen-free environment. This is particularly important for bacteria that use the Wood-Ljungdahl pathway, which involves the reduction of carbon dioxide to produce energy.

**Correct Answer:** D.