The least number of trees planted is the least common multiple (LCM) of 3 and 10. Because 3 and 10 have no factors in common, that value is their product, 3·10 = 30.
If group B plants 30 trees, they will have planted 3 clusters.
It’s A and B because multiplying the grouping numbers does not change the product. (4x6) x7 = 168 and 4 x (6x7) also equal to 168. Same thing as B but the number just change order.