**Core Concept**
The primary teeth, also known as baby teeth, play a crucial role in the development of the jaw and facial structure. The primary dentition is a set of 20 deciduous teeth that erupt in the first six years of life and begin to fall out around the age of six, making way for the permanent dentition.

**Why the Correct Answer is Right**
During the first 12 years of life, humans spend a significant amount of time chewing with primary teeth. This period is essential for the proper development of the jaw and facial muscles. The primary teeth are designed to be smaller and more fragile than permanent teeth but are crucial for learning to chew and speak properly. The correct answer is based on the average time children spend with primary teeth, which is approximately 6 years.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it is too high, and primary teeth typically begin to fall out around the age of 6.
* **Option B:** This option is incorrect because it is too low, and the primary dentition plays a crucial role in the first 6 years of life.
* **Option C:** This option is incorrect because it is not a specific fraction, and the question asks for a fraction of life spent chewing with primary teeth.

**Clinical Pearl / High-Yield Fact**
The primary dentition is responsible for learning to chew and speak properly, and any issues with primary teeth can affect the development of the permanent dentition.

**Correct Answer:** **D. 50%**