The element of the set can be disintegrated as

1 = 1²

4 = 2²

9 = 3²

…..

.

.

.

. 100 = 10²

The set in the set-builder form is

A= {x:x = n², n ∈N, 1≤n ≤10}

Alternatively

A = {x:x is the square of natural number less than or equal to 10}