Explanation

Discrepancy Note: The official UPSC answer key marked Option A (1, 2 and 3 only), excluding vertical farming. However, standard agronomic literature and leading environmental institutions classify all four practices as eco-friendly sustainable agriculture, making Option D (1, 2, 3 and 4) the universally accepted substantive answer.

– 1. Crop diversification: Cultivating diverse cropping sequences disrupts insect-pest cycles, preserves soil microbial biodiversity, and curtails reliance on chemical agrochemicals.
– 2. Legume intensification: Incorporating nitrogen-fixing leguminous crops naturally enriches soil fertility via biological nitrogen fixation, reducing synthetic nitrogenous fertilizer consumption.
– 3. Tensiometer use: Measuring soil moisture tension with tensiometers optimizes irrigation scheduling, preventing waterlogging, soil salinization, and groundwater depletion.
– 4. Vertical farming: Growing crops in vertically stacked layers under controlled environmental systems (hydroponics/aquaponics) maximizes land-use efficiency, recycles up to 90-95% of water, and eliminates pesticide runoff into natural waterways.

Therefore, all four practices represent eco-friendly agricultural techniques — Option D.