**Core Concept:** Mouthing, stranger anxiety, and food likes and dislikes are all developmental milestones in infants that are important indicators of their age and overall development.

**Why the Correct Answer is Right:** The correct answer is **D**. Mouthing is a normal developmental behavior in infants, where they explore their environment by mouthing objects. However, stranger anxiety is a specific aspect of social-emotional development that occurs around 9-12 months of age. Food likes and dislikes are also important as they reflect the child's nutritional needs and preferences.

Infants at approximately 6 months of age, such as the one presented in this question, are typically still dependent on their caregivers for nutrition and do not have a clear preference for food. As the child moves towards 12-18 months of age, they begin to develop food likes and dislikes, which indicates the onset of oral exploration and autonomy in feeding.

**Why Each Wrong Option is Incorrect:**

A. Mouthing occurs in infants of all age groups, not just 6 months, and stranger anxiety is a 9-12 months old milestone.
B. While food preferences emerge around 6-12 months, the lack of stranger anxiety and clear food likes/dislikes make this infant's age inappropriate.
C. Similar to option A, mouthing happens at various ages, and the lack of stranger anxiety and clear food preferences indicates an older or younger age than 6 months.
D. The combination of appropriate mouthing, lack of stranger anxiety, and development of food likes/dislikes suggests that the infant is around 6-7 months old.

**Clinical Pearl:** Tracking milestones like mouthing, stranger anxiety, and food preferences can provide insights into the overall development of an infant. Monitoring these milestones can help healthcare providers assess if a child is developing within the expected range and provide appropriate guidance to parents.

**Correct Answer Explanation:** The correct answer, **D**, reflects the typical development of an infant at around 6-7 months of age. During this period, infants will exhibit mouthing behavior (exploring objects in the mouth), have not developed stranger anxiety (not showing fear or unease when encountering unfamiliar people), and are starting to develop food preferences (beginning to show a liking or disliking for specific foods).

In conclusion, the correct answer, 6-7 months, is appropriate as it aligns with the developmental milestones described: mouthing, stranger anxiety, and food preferences. Monitoring and tracking these milestones can assist healthcare providers in assessing a child's overall development and provide guidance to parents regarding their child's progress.