Answer:
3. A quadrilateral with each side length 4 cm
Explanation:
We need to understand what is a unique polygon?
It means all regular polygons with the same number of sides are similar.
1.A triangle with angles 30 degrees, 60 degrees, and 90 degrees
It is an right triangle, so the 3 sides are not simillar, so it is wrong
2. A triangle with side lengths 5 cm, 4 cm, and 3 cm
It is a triangle with the 3 sides are not simillar, so it is wrong
3. A quadrilateral with each side length 4 cm
It is an square with the 4 sides are simillar, so it is true
4. A triangle with side lengths 7 cm and 8 cm and a 40-degree angle
It is a triangle with the 3 sides are not simillar, so it is wrong
Hence, we choose 3.