**Core Concept:** Understanding basic principles of stacking and geometry. In this case, we are concerned with the number of sides of the base cube.

**Why the Correct Answer is Right:** The correct answer is **D**. If we consider a cube as a three-dimensional shape with equal sides, the base of the tower will have four sides. Since the question is asking for the condition when the child can make a tower of 4 cubes, the base must have four equal sides. In all other options (A, B, C), the base would have either three or five sides, making it impossible to stack four cubes on top.

**Why Each Wrong Option is Incorrect:**

A. Using a square base (three-sided base) would result in an unequal height for each cube, making the structure unstable.
B. A pentagonal base (five-sided base) has unequal sides, which would also make the structure unstable.
C. Using a square base (three-sided base) again results in unequal height for each cube, making the structure unstable.

**Why the Correct Answer is Right (again):**

A **correctly designed tower** would be stable and have equal sides. In this case, a cube has four equal sides, ensuring stability when stacked on top of one another.

**Clinical Pearl or High-Yield Fact:** The concept of four-sided bases (tetrahedrons) is essential in understanding stability and geometry in various medical scenarios, particularly in designing structures like scaffolding and surgical splints. Additionally, it highlights the importance of considering the stability of structures when designing medical devices or surgical procedures.