**Core Concept**
The mixed dentition classification of malocclusion involves assessing the alignment of both primary and permanent teeth in children. This classification is crucial for early orthodontic diagnosis and treatment planning. The correct classification method should consider the unique characteristics of mixed dentition, including the presence of both primary and permanent teeth.

**Why the Correct Answer is Right**
The Flush Terminal Plane (FTP) is used in the mixed dentition classification of malocclusion. This method assesses the alignment of the teeth by examining the terminal plane of the primary molars and the terminal plane of the permanent molars. The FTP is an important indicator of the overall occlusal relationship in mixed dentition. When the terminal planes are flush, it indicates a normal occlusal relationship, whereas any deviation from this indicates a malocclusion.

**Why Each Wrong Option is Incorrect**
**Option A:** The Angle's classification is primarily used for adult dentition and does not take into account the unique characteristics of mixed dentition. It is not suitable for classifying malocclusions in children.

**Option B:** Simon's orbital plane is not related to the classification of malocclusions in mixed dentition. It is a concept used in orthodontic diagnosis, but it is not relevant to this specific classification method.

**Option D:** Since only one of the options (C. Flush terminal plane) is correct, option D cannot be correct.

**Clinical Pearl / High-Yield Fact**
When classifying malocclusions in mixed dentition, it's essential to remember that the Flush Terminal Plane method is a useful indicator of the overall occlusal relationship. This method can help identify potential issues early on, allowing for timely orthodontic intervention.

**✓ Correct Answer: C. Flush terminal plane**