Land equivalent ratio is useful index under:

✅ EXPLANATION: Land equivalent ratio (LER) is a useful index under both additive series and replacement series.
🔹 Additive series: In additive series, intercrops are grown in different rows or plots, and the yields of individual crops are added to determine the LER.
🔹 Replacement series: In replacement series, intercrops are grown in the same row or plot, and the yield of the intercropping system is compared with the yields of individual crops grown separately on the same area of land.

📌Other Options and Explanations:
🔹 Land equivalent ratio (LER): A measure of the productivity of a multiple cropping system in comparison with a monoculture system.
🔹 Multiple cropping system: A cropping system in which two or more crops are grown at the same time in the same field.