## **Core Concept**
The question assesses the developmental milestones of a child, particularly focusing on gross motor skills, fine motor skills, and language development. These milestones are crucial in pediatric assessments to determine a child's age and developmental progress.

## **Why the Correct Answer is Right**
At a certain age, children are expected to achieve specific developmental milestones.
- Climbing with alternate steps indicates improved gross motor skills.
- Building a tower of 8-9 cubes shows advanced fine motor skills and hand-eye coordination.
- Saying 'I' but not his name, and not being able to state his age and sex, points to specific stages in language and cognitive development.
These skills align with a child around **2 years of age**, who demonstrates these abilities as part of normal developmental progression.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Typically corresponds to a younger age. At this age, children might not have achieved the described milestones, such as building a tower of 8-9 cubes or using 'I' in sentences.
- **Option C:** Corresponds to an older age where children are expected to have surpassed these milestones, possibly being able to state their name, age, and sex, and having more advanced building skills.
- **Option D:** This option suggests an even older age, where the described abilities would be considered delayed.

## **Clinical Pearl / High-Yield Fact**
A key developmental milestone to remember is that by **18 months**, children often say 5-20 words and start combining two words together. By **2 years**, they typically have a vocabulary of around 50 words and start forming simple sentences, like "mama go." Also, at 2 years, children can usually build a tower of 6-9 cubes, which aligns with the description provided.

## **Correct Answer: B. 2 years.**