**Core Concept**
The tower test is a developmental milestone that assesses fine motor skills, hand-eye coordination, and problem-solving abilities in children. It is a part of the Denver Developmental Screening Test, which evaluates cognitive, motor, and language development in infants and young children.

**Why the Correct Answer is Right**
Children typically develop the necessary fine motor skills and hand-eye coordination to build a tower of 9 cubes around 30 months of age. This milestone is an indicator of normal cognitive and motor development. The ability to build a tower of 9 cubes requires the child to have sufficient strength, dexterity, and problem-solving skills to manipulate the cubes and balance the structure.

**Why Each Wrong Option is Incorrect**
**Option A:** 24 months is too early for a child to build a tower of 9 cubes, as most children are still developing their fine motor skills and hand-eye coordination at this age.
**Option C:** 36 months is still within the normal range, but most children can build a tower of 9 cubes by 30 months, making this option incorrect.
**Option D:** 48 months is too late, as children typically master this skill well before the age of 4.

**Clinical Pearl / High-Yield Fact**
Remember that developmental milestones are not absolute and can vary depending on the child's individual development and cognitive abilities. A delay in a specific milestone does not necessarily indicate a developmental disorder, but rather warrants further evaluation and assessment.

✓ Correct Answer: B. 30 months