Explanation:

- The perimeter of a square is calculated as four times the length of one side.

- According to the problem, the perimeter is 90 cm more than one side's length.

- Let the side length be $x$.

- The equation becomes: $4x=x+90$.

- Solving gives: $4x-x=90$, so $3x=90$.

- This results in $x=30$.

Options:

1. 25 cm

- Wrong, doesn’t satisfy $3x=90$.

2. 30 cm

- Correct, satisfies $x=30$.

3. 60 cm

- Wrong, 3 times 60 doesn’t add up to 90.

4. 45 cm

- Wrong, 3 times 45 doesn’t equal 90.