**Core Concept**
At 36 months, children typically achieve significant milestones in gross motor development, including improved balance and coordination. Riding a tricycle is a critical milestone that reflects these developments, enabling children to move independently and explore their environment.

**Why the Correct Answer is Right**
Riding a tricycle requires the integration of gross motor skills, including balance, coordination, and pedaling. At 36 months, children typically develop sufficient balance and coordination to ride a tricycle, which is a crucial step in learning to ride a bike. This milestone also reflects the child's ability to follow instructions, practice, and refine their skills. The development of these skills is essential for further gross motor milestones, such as riding a bike with training wheels and eventually without.

**Why Each Wrong Option is Incorrect**
**Option A:** Drawing a rectangle is a fine motor skill milestone, typically achieved around 2-3 years of age, which is before the age of 36 months.
**Option B:** Building a tower of 20 cubes is a cognitive and fine motor skill milestone, often achieved around 3-4 years of age, which is beyond the age of 36 months.
**Option D:** Skipping is a gross motor skill milestone, but it is typically achieved around 3-4 years of age, when children have developed sufficient balance and coordination.

**Clinical Pearl / High-Yield Fact**
Remember that every child develops at their own pace, and some may achieve certain milestones earlier or later than their peers. However, significant delays in gross motor development, such as not being able to ride a tricycle by 36 months, may warrant further evaluation and assessment.

**✓ Correct Answer: C. Riding a tricycle**