1. **Problem:** Find the length of a side of a square whose perimeter is 1600 cm. 2. **Formula:** The perimeter $P$ of a square is given by $$P = 4 \times \text{side length}$$ 3. **Step:** Let the side length be $s$. Then, $$4s = 1600$$ 4. **Step:** To find $s$, divide both sides by 4: $$\frac{\cancel{4}s}{\cancel{4}} = \frac{1600}{4}$$ $$s = 400$$ 5. **Answer:** The length of each side of the square is $400$ cm. 6. **Choose the best answer:** a. 400cm This means the correct choice is **a. 400cm**.