• Layout of Straight Furrows in Surface Irrigation:
• Furrow Alignment:
• Straight Furrows: Aligned and laid directly down the prevailing slope of fields with uniform, gentle gradients (typically $0.1\%$ to $0.5\%$, rarely exceeding $1.0\%$).
• Contour Furrows: On undulating, steep terrain ($\gt 2 - 3\%$), straight furrows would cause high water velocities and severe soil erosion; furrows are therefore laid across the slope following natural topographic contours.
• Conclusion: Laid down the prevailing slope (Option 4).

Correct option: (4) Laid down the prevailing slope