**Core Concept:**
The development of cognitive and fine motor skills is essential for a child to perform tasks like building a tower with cubes. These skills mature over time, with increasing complexity.

**Why the Correct Answer is Right:**
At around 3 years of age, a child begins to develop the ability to grasp objects firmly and manipulate them into desired positions, which is crucial for stacking cubes. This milestone marks the beginning of the development of fine motor skills.

**Why Each Wrong Option is Incorrect:**
A. **Option A (1 year):** A child at this age is not able to hold objects firmly and manipulate them accurately, making stacking cubes impossible.
B. **Option B (2 years):** While a child can grasp objects at this age, their hand-eye coordination and fine motor skills are not developed enough to stack cubes accurately.
C. **Option D (5 years):** Fine motor skills reach a peak around 4-5 years of age, but stacking cubes requires more than just motor skills, like cognitive understanding and spatial awareness.

**Clinical Pearl / High-Yield Fact:**
The ability to stack cubes is a marker of cognitive development, particularly problem-solving skills and spatial awareness. It helps evaluate the child's development and overall readiness for learning tasks that require fine motor skills.

**Correct Answer:** C. 5 years

By around 5 years of age, a child typically possesses the necessary fine motor skills, cognitive understanding, and spatial awareness to build a stable tower with cubes.