**Core Concept**
The question is testing the child's developmental milestone of gross motor skills, specifically the ability to stack and balance objects, known as **tower building**. This skill is a crucial aspect of fine and gross motor development, reflecting the maturation of the child's hand-eye coordination and spatial awareness.

**Why the Correct Answer is Right**
Tower building is a complex skill that requires the integration of multiple motor and cognitive abilities. Around the age of 15-18 months, children typically develop the necessary hand-eye coordination, balance, and problem-solving skills to successfully build a tower of 3 cubes. This milestone is an important indicator of normal development, and its achievement is often used as a benchmark in pediatric assessments.

**Why Each Wrong Option is Incorrect**
* **Option A:** The age range for tower building is generally considered to be between 15-18 months, so an age below this range would be considered developmentally delayed.
* **Option B:** The correct answer is not 4 years, as tower building is typically achieved before the age of 2.
* **Option C:** This option is not provided, as the question only includes options A, B, and D.

**Clinical Pearl / High-Yield Fact**
It's essential to note that developmental milestones can vary across children, and some may achieve certain skills earlier or later than their peers. However, if a child is significantly delayed in achieving a particular milestone, it may indicate an underlying developmental disorder or neurological issue.

**Correct Answer: B. 18 months**