Which option correctly conveys the idea that one person is not as tall as another using "as"?

The correct option effectively conveys the idea that one person is not as tall as another by explicitly stating the comparative relationship between the two individuals. When one says "His brother is taller than he is," it directly indicates that the brother's height surpasses that of the other person, which inherently implies that the first person is not as tall.

This structure focuses on the comparative aspect without ambiguity, making it clear and straightforward. The use of the word "taller" serves to establish that there is a difference in height, aligning directly with the notion of one person being less tall than another.

In contrast, while other options might also suggest a comparison, they may do so in less direct ways or convey different nuances. For example, stating that "He is not as tall as his brother" also communicates the intended comparison but is framed differently, potentially making it less immediate than a direct comparison.