1. **State the problem:** We need to find the area of triangle $\triangle KLM$ with base $KM = 42$ feet and height from $L$ to $KM$ as $24$ feet. 2. **Formula for area of a triangle:** $$\text{Area} = \frac{1}{2} \times \text{base} \times \text{height}$$ 3. **Apply the values:** $$\text{Area} = \frac{1}{2} \times 42 \times 24$$ 4. **Calculate step-by-step:** $$= \frac{1}{2} \times 1008$$ $$= 504$$ 5. **Conclusion:** The area of $\triangle KLM$ is $504$ square feet. **Answer: A. 504**