**Core Concept**
The DMF index is a measure of dental caries experience used to assess the prevalence of dental decay. It is a widely used metric in pediatric dentistry to evaluate the oral health of children.

**Why the Correct Answer is Right**
The correct answer is related to the adaptation of the DMF index for primary teeth. The DMF index for primary teeth includes decayed (D), missing (M), and filled (F) primary teeth. The index is used to monitor the dental health of children and is especially relevant in pediatric dentistry. The inclusion of primary teeth in the DMF index allows for a more accurate assessment of dental caries experience in children.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the correct adaptation of the DMF index for primary teeth.
**Option B:** This option is incorrect because it does not accurately describe the DMF index for primary teeth.
**Option C:** This option is incorrect because it does not provide a valid adaptation of the DMF index for primary teeth.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the DMF index for primary teeth is used to monitor the dental health of children and is especially relevant in pediatric dentistry. The inclusion of primary teeth in the DMF index allows for a more accurate assessment of dental caries experience in children.

**Correct Answer: D. Filled primary teeth are included in the DMF index**