Which of the following is most appropriate for blank “a”?

Paragraph for question 17 to 20 Agroforestry integrates trees with crops and/or animals to create sustainable agricultural systems, enhancing ecological and economic benefits. The three main types include agrisilvicultural systems, which combine trees with crops, a systems, integrating trees with pastures or animals, and b systems, which involve animals, trees, and crops together. These systems contribute to improved biodiversity, soil health, and resource efficiency, making them valuable for modern agricultural practices and environmental conservation.