**Core Concept**
The number of teeth present in a child's mouth at a specific age is an indicator of dental development and overall health. By 7 years of age, most children have a mix of primary and permanent teeth, which is an essential milestone in pediatric dentistry.

**Why the Correct Answer is Right**
Typically, by 7 years of age, a child has a total of 60 teeth, comprising 20 primary incisors, 20 primary molars, 8 primary canines, and 12 permanent first molars. This dental arrangement is a result of the sequential eruption of primary and permanent teeth, which starts at birth and continues until the age of 12-13 years.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not account for the presence of permanent first molars, which typically erupt between 6-7 years of age.
* **Option B:** This option is incorrect because it underestimates the number of primary incisors present in a 7-year-old child's mouth.
* **Option C:** This option is incorrect because it does not accurately represent the total number of teeth present in a 7-year-old child's mouth.

**Clinical Pearl / High-Yield Fact**
One crucial point to remember is that the eruption time of teeth can vary significantly among children, and some may have a full complement of permanent teeth by the age of 6, while others may still have primary teeth at the age of 8.

**Correct Answer: D. 60**