The set $\{1, 4, 9, 16, 25, \ldots\}$ in set-builder form is:

The pattern $1, 4, 9, 16, 25, \ldots$ is $1^{2}, 2^{2}, 3^{2}, 4^{2}, 5^{2}, \ldots$ — i.e. the **squares of natural numbers**. In set-builder form: $\{x : x = n^{2}, n \in \mathbb{N}\}$. Equivalently: $\{x : x \text{ is the square of a natural number}\}$. - Trap A gives multiples of $4$: $\{4, 8, 12, 16, 20, \ldots\}$ — different set. - Trap C gives evens: $\{2, 4, 6, 8, \ldots\}$. - Trap D gives $\{3, 5, 7, 11, \ldots\}$. NCERT Class 11, Example 3.