1. **State the problem:** We have a spring stretched by a force (weight) of 100 N causing an extension of 0.5 m. We need to find the spring constant $k$. 2. **Formula used:** Hooke's Law states that the force $F$ applied to stretch or compress a spring is proportional to the displacement $x$: $$F = kx$$ where $k$ is the spring constant. 3. **Rearrange the formula to solve for $k$:** $$k = \frac{F}{x}$$ 4. **Substitute the given values:** $$k = \frac{100}{0.5}$$ 5. **Calculate:** $$k = 200$$ 6. **Interpretation:** The spring constant is 200 N/m. **Answer:** d. 200 N/m