• Remote Sensing in Plant Disease Diagnosis:
• Spectral Bands Utilized:
• Visible Spectrum ($400-700\text{ nm}$): Healthy green vegetation strongly absorbs blue ($450\text{ nm}$) and red ($650\text{ nm}$) wavelengths for photosynthesis, reflecting green light. Diseased leaves lose chlorophyll, leading to increased reflectance in the red and blue regions.
• Near-Infrared Spectrum (NIR, $700-1100\text{ nm}$): Healthy internal leaf architecture (spongy mesophyll) scatters and reflects $\gt 50\%$ of incident NIR light. Pathogen invasion disrupts cell walls and collapses intercellular air spaces, drastically reducing NIR reflectance.
• Normalized Difference Vegetation Index (NDVI): Relies on combining reflectance in both visible and infrared regions ($NDVI = \frac{NIR - Red}{NIR + Red}$) to detect pre-visual and visual disease symptoms.
• Conclusion: Both visible and infrared wavelengths are critical (Option 3).

Correct option: (3) Both visible and infrared