The correct answer is (d). Alley cropping is a cropping system where crops are grown in the inter spaces between rows of planted trees. The trees provide shade and windbreaks for the crops, and their leaves can be used as mulch or compost. Alley cropping can help to improve soil fertility, reduce erosion, and increase crop yields.
(a) Relay cropping is a cropping system where a second crop is planted after the first crop has been harvested. This can be done to extend the growing season or to increase crop yields.
(b) Mixed cropping is a cropping system where two or more crops are grown together in the same field. This can be done to improve soil fertility, reduce pests and diseases, and increase crop yields.
(c) Inter-cropping is a cropping system where two or more crops are grown in the same field, but in different rows or areas. This can be done to improve soil fertility, reduce pests and diseases, and increase crop yields.