**Core Concept**
The "Ugly duckling" stage of the transitional dentition is a critical period in the development of the permanent dentition. It is characterized by the presence of both primary and permanent teeth in the same dental arch, leading to a mixed dentition. During this stage, the primary teeth are gradually being replaced by the permanent teeth.

**Why the Correct Answer is Right**
The "Ugly duckling" stage is marked by the presence of both primary and permanent teeth, which can lead to dental crowding, malalignment, and other orthodontic issues. The primary teeth are gradually being resorbed and replaced by the permanent teeth, which is a complex process involving the coordinated action of various cellular and molecular mechanisms. The permanent teeth are initially smaller than the primary teeth and gradually grow to their full size, leading to a change in the occlusal relationship between the teeth.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the "Ugly duckling" stage is indeed characterized by the presence of both primary and permanent teeth.
* **Option B:** This option is incorrect because the primary teeth are gradually being resorbed and replaced by the permanent teeth during this stage.
* **Option C:** This option is incorrect because the permanent teeth are initially smaller than the primary teeth and gradually grow to their full size.

**Clinical Pearl / High-Yield Fact**
The "Ugly duckling" stage of the transitional dentition is a critical period for orthodontic evaluation and treatment planning. Early identification of orthodontic issues during this stage can help prevent long-term dental and facial problems.

**Correct Answer: None of the above.**