**Core Concept**
Developmental milestones in infants and toddlers follow a predictable sequence, with motor, cognitive, and fine motor skills advancing in a stage-specific manner. At 10 months, children typically achieve key milestones in gross and fine motor development, including standing, object manipulation, and simple play behaviors.

**Why the Correct Answer is Right**
At 10 months, a child can typically stand alone (A), play peek-a-boo (B), and pick up small objects using thumb and index finger (C), which reflects emerging hand-eye coordination and grasping ability. However, building a tower of 3–4 cubes is a milestone that usually occurs around 15–18 months, not at 10 months. This skill requires advanced coordination, balance, and spatial awareness, which are not yet fully developed in a 10-month-old.

**Why Each Wrong Option is Incorrect**
Option A: Standing alone is a well-established milestone at 10 months, achieved through improved muscle strength and balance.
Option B: Peek-a-boo is a foundational social interaction skill, typically mastered by 10 months.
Option C: Picking up a pellet with thumb and index finger reflects the pincer grasp, which develops by 9–10 months.

**Clinical Pearl / High-Yield Fact**
Remember the "10-month rule": by 10 months, a child should stand, play peek-a-boo, and grasp small objects with precision. Tower-building is a marker of later development and should not be expected before 15 months.

✓ Correct Answer: D. Build a tower of 3–4 cubes