**Core Concept**
The child's developmental age is being assessed using the developmental milestone of tower building, which is a marker of fine motor skills and problem-solving abilities. This milestone is typically achieved by children around 3-4 years of age, as per the Denver Developmental Screening Test.

**Why the Correct Answer is Right**
The child's ability to build a tower of 5 cubes indicates a certain level of cognitive and motor development. Children typically progress from building a single block to stacking 2-3 blocks, and then to 4-5 blocks around the age of 3-4 years. This milestone is a reflection of the child's hand-eye coordination, problem-solving skills, and ability to plan and execute a task.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because building a tower of 1-2 cubes is a milestone achieved by children around 1-2 years of age, which is significantly lower than the developmental age indicated by building a tower of 5 cubes.
**Option B:** This option is incorrect because building a tower of 6-7 cubes is a milestone achieved by children around 4-5 years of age, which is higher than the developmental age indicated by building a tower of 5 cubes.
**Option C:** This option is incorrect because building a tower of 3-4 cubes is a milestone achieved by children around 2-3 years of age, which is lower than the developmental age indicated by building a tower of 5 cubes.

**Clinical Pearl / High-Yield Fact**
The Denver Developmental Screening Test is a widely used tool for assessing developmental milestones in children. It evaluates various skills, including gross motor, fine motor, language, and problem-solving abilities.

**Correct Answer:** C. 2 years 6 months