**Core Concept**
The question assesses the child's fine motor skills and cognitive development, specifically their ability to stack and balance objects, a classic indicator of gross and fine motor milestones in early childhood.

**Why the Correct Answer is Right**
Stacking of 4 cubes is a significant developmental milestone, and children typically achieve this around 1.5 years of age. This skill requires the integration of fine motor skills, hand-eye coordination, and problem-solving abilities. The child must learn to balance the cubes, adjust their grip, and apply the correct amount of pressure to achieve stability. This process enhances their hand strength, dexterity, and cognitive abilities.

**Why Each Wrong Option is Incorrect**
**Option A:** 1 year is too early for stacking 4 cubes, as children usually start with single cubes around 9-12 months and progress to stacking 2 cubes at 12-14 months.
**Option C:** 3 years is too late for this specific milestone, as most children have developed more complex fine motor skills by this age.
**Option D:** 4 years is even later than option C and is not a typical age for achieving this skill.

**Clinical Pearl / High-Yield Fact**
Keep in mind that the stacking of cubes is a classic example of a developmental milestone that can be used to assess a child's fine motor and cognitive development. This skill is an essential part of the Denver Developmental Screening Test (DDST) and the Bayley Scales of Infant Development (BSID).

✓ Correct Answer: B. 1&1/2 years