**Core Concept**
The serial extraction philosophy is an approach to pediatric dentistry that emphasizes the gradual removal of primary teeth to prevent crowding and orthodontic problems in the permanent dentition. This concept is rooted in the understanding of the importance of primary teeth in guiding the eruption and positioning of permanent teeth.

**Why the Correct Answer is Right**
Nancy is credited with developing the serial extraction philosophy in the United States. Her approach involves the selective removal of primary teeth to create space for the eruption of permanent teeth, thereby reducing the risk of crowding and orthodontic complications. This method requires careful planning and monitoring to ensure that the extraction sequence is optimal for each patient's individual needs.

**Why Each Wrong Option is Incorrect**
**Option A:** Kjellgren is not associated with the serial extraction philosophy in the USA. Instead, Kjellgren's approach emphasizes the removal of primary teeth in a more aggressive manner, which is not in line with the principles of serial extraction.
**Option C:** Dewel is a well-known figure in pediatric dentistry, but he is not credited with developing the serial extraction philosophy. Dewel's contributions to the field are more focused on the area of pediatric oral pathology.
**Option D:** Hotz is not a recognized figure in the context of the serial extraction philosophy in the USA.

**Clinical Pearl / High-Yield Fact**
The serial extraction philosophy is a valuable tool in pediatric dentistry, but its application requires careful consideration of each patient's unique needs and oral anatomy. A thorough understanding of the underlying principles and careful planning are essential to ensure optimal outcomes.

**✓ Correct Answer:** B. Nancy