## **Core Concept**
The question assesses the developmental milestones of infants, specifically focusing on the psychological and social development as outlined by various pediatric and psychological guidelines. The key milestones mentioned here include mouthing objects, showing likes and dislikes, and the absence of stranger anxiety.

## **Why the Correct Answer is Right**
The developmental milestone of mouthing objects is typically seen in infants around 6 to 9 months of age. Showing likes and dislikes is also a part of social and emotional development that starts to become apparent during this period. Stranger anxiety, which is the fear of strangers and can be a significant milestone, usually develops a bit later, around 9 months of age. Given that the child in the question has not yet developed stranger anxiety but is mouthing objects and showing preferences, this places the child's age likely around 6 to 8 months.

## **Why Each Wrong Option is Incorrect**
- **Option A:** If the child were 3 months old, they would not yet be mouthing objects or showing clear likes and dislikes as these are developmental milestones achieved later.
- **Option B:** A 12-month-old child would likely have developed stranger anxiety by this age, making this option incorrect given the details in the question.
- **Option D:** A 18-month-old child would definitely have developed stranger anxiety and would be exhibiting more advanced language and social skills than those described.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that developmental milestones can vary among children, but they generally follow a predictable pattern. The absence of stranger anxiety in a child who is otherwise developing typically might suggest an age under 9 months. A helpful mnemonic for developmental stages is not directly applicable here, but recalling that stranger anxiety peaks around 9-12 months helps in eliminating options.

## **Correct Answer:** B. 6 months.