**Core Concept**
The cervical portions of primary molars exhibit unique characteristics due to their smaller size and rapid growth pattern compared to permanent teeth. This results in differences in the morphology and structure of the crowns and roots.

**Why the Correct Answer is Right**
The crowns and roots of primary molars are more conical in shape at their cervical portions. This is because the primary molars have a smaller root canal system and less complex root anatomy compared to permanent molars. The conical shape allows for easier resorption and shedding of the primary teeth. The enamel and dentin at the cervical portions are also thinner in primary molars, making them more susceptible to wear and caries.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the crowns and roots of primary molars are not more flattened at their cervical portions. The conical shape is a distinct characteristic of primary molars.
* **Option B:** This option is incorrect because the crowns and roots of primary molars are not more complex at their cervical portions. The primary molars have a simpler root anatomy compared to permanent molars.
* **Option D:** This option is incorrect because the crowns and roots of primary molars are not more bulbous at their cervical portions. The conical shape is a characteristic feature of primary molars.

**Clinical Pearl / High-Yield Fact**
It's essential for dentists and pediatricians to recognize the unique characteristics of primary molars, including their conical shape, thinner enamel and dentin, and simpler root anatomy. This knowledge helps in diagnosing and treating dental caries and other issues in pediatric patients.

**Correct Answer: C. conical**