Eco-friendly agriculture refers to farming practices that are sustainable and cause minimal harm to the environment.

Crop diversification is one such practice. It involves growing a variety of crops in the same area over different seasons. This method enhances soil health, reduces the need for excessive water and fertilizers, and boosts biodiversity in the ecosystem. By rotating crops, it also helps in preventing soil depletion and improves resilience against pests and diseases.

Legume intensification is another eco-friendly practice. Legumes are known to improve soil fertility by fixing nitrogen from the atmosphere. This reduces the need for synthetic fertilizers, making agriculture more sustainable.

Tensiometer is a useful tool in efficient water management. It measures the water tension in soil, which helps in determining the availability of water for crops. By ensuring that water is used efficiently, it reduces waste and promotes sustainable water usage, making it an eco-friendly farming practice.

Vertical farming involves growing crops in vertically stacked layers, often using advanced techniques like hydroponics, aeroponics, and aquaponics, which do not require soil. While this method can increase production and reduce land usage, it requires high energy consumption and specialized technology. Hence, it is not widely considered as an eco-friendly practice.

Therefore, the correct answer is A. 1, 2, and 3 only.