**Core Concept**
The question is testing the developmental milestone of ambulation and gross motor skills in infants and toddlers. Gross motor skills development is a crucial aspect of pediatric development, and understanding the sequence and timing of these milestones is essential for pediatricians and healthcare providers.

**Why the Correct Answer is Right**
Typically, infants start to develop their gross motor skills around 6-9 months of age. Climbing steps with alternate feet is a significant milestone achieved around 12-14 months of age. This skill requires coordination, balance, and strength, which are refined through various gross motor activities such as crawling, standing, and walking. The development of alternate foot climbing is a sign of advanced gross motor skills and is an essential part of a child's physical development.

**Why Each Wrong Option is Incorrect**
* **Option A:** 9-10 months - This is too early for alternate foot climbing, as most infants start to develop this skill around 12-14 months.
* **Option B:** 15-16 months - This is too late, as most children have already achieved this milestone by 14 months.
* **Option C:** 18-20 months - This is significantly later than the expected age for alternate foot climbing.

**Clinical Pearl / High-Yield Fact**
Keep in mind that developmental milestones can vary significantly among children, and a delay of more than 2-3 months in achieving a milestone may indicate a potential developmental issue. Regular monitoring and assessment by a pediatrician are essential in identifying and addressing any potential developmental concerns.

**Correct Answer:** C. 12-14 months.