**Core Concept**
The question is assessing the child's gross motor skills and cognitive development, specifically the milestone of tower building with cubes. This task requires hand-eye coordination, fine motor control, and problem-solving abilities, which are essential for neurological development.

**Why the Correct Answer is Right**
The child's ability to build a tower of 7 cubes is a marker of advanced fine motor skills and cognitive development. Typically, children achieve this milestone around 18-24 months of age. This skill is closely related to the development of the prefrontal cortex and the maturation of the corticospinal tract. The child's brain is refining its ability to plan, execute, and adapt movements, which is crucial for complex tasks like tower building.

**Why Each Wrong Option is Incorrect**
**Option A:** This age is too early for the child to build a tower of 7 cubes, as it typically requires more refined fine motor skills and cognitive abilities.
**Option B:** This age is too late for the child to build a tower of 7 cubes, as most children achieve this milestone before 2 years of age.
**Option C:** This age is not specific enough, as it does not take into account the child's individual developmental pace.

**Clinical Pearl / High-Yield Fact**
Remember that the development of fine motor skills and cognitive abilities in children is closely linked to the maturation of the brain's motor and executive function networks. Delays in these milestones can indicate underlying neurological or developmental disorders.

**Correct Answer: C. 18-24 months**