## Core Concept
Gross motor skills in children develop in a predictable sequence, with specific milestones achieved within certain age ranges. Climbing steps with alternate feet is a significant gross motor skill milestone that reflects a child's coordination, balance, and lower limb strength.

## Why the Correct Answer is Right
The milestone of climbing steps with alternate feet is typically achieved by children around 24 months of age. This skill requires a combination of strength, coordination, and balance. At this stage, children's neuromuscular development enables them to perform more complex movements, such as alternating their feet while climbing stairs.

## Why Each Wrong Option is Incorrect
* **Option A:** 18 months - This is too early for most children to achieve the milestone of climbing steps with alternate feet. While some children may start to show early signs of this ability, it is generally not mastered at this age.
* **Option B:** 30 months - This is later than the typical age range for achieving this milestone. Most children should be able to climb steps with alternate feet by 24 months.
* **Option D:** 36 months - This is significantly later than the expected age for this milestone. Children who have not achieved this skill by 36 months may require further evaluation for potential developmental delays.

## Clinical Pearl / High-Yield Fact
A useful clinical correlation is that by 24 months, children are expected to achieve several gross motor milestones, including climbing up and down stairs with support and kicking a ball forward. Parents and clinicians can use these milestones to monitor developmental progress.

## Correct Answer: C. 24 months.