**Core Concept**
The question refers to a developmental milestone in children, specifically the ability to stack objects, which is an indicator of fine motor skills, hand-eye coordination, and cognitive development. This ability is often assessed in the context of the Denver Developmental Screening Test.

**Why the Correct Answer is Right**
The ability to stack three cubes is a fine motor skill milestone that typically emerges in children around 15-18 months of age. This skill requires the child to coordinate their hand movements, grasp objects, and balance them, which are essential components of motor development. The exact age may vary depending on the individual child, but 15-18 months is the generally accepted range for this milestone.

**Why Each Wrong Option is Incorrect**
**Option A:** 6 months - This is too early for the child to demonstrate fine motor skills necessary for stacking objects.
**Option B:** 24 months - This is too late, as most children can stack three cubes by 18 months of age.
**Option C:** 12 months - While some children may start to show interest in stacking objects at 12 months, they typically need a bit more time to develop the necessary fine motor skills.

**Clinical Pearl / High-Yield Fact**
Keep in mind that developmental milestones can vary significantly among children, and some may need more time to master certain skills. It's essential to monitor a child's progress and address any concerns with a pediatrician if developmental delays are suspected.

**Correct Answer: B. 24 months.**