The reason why the trees of tropical rainforest have buttress roots is because these roots have to bear the mechanical load of the hardwoods. Buttress roots are large and wide roots that extend from the base of the trunk of a tree and provide extra support to the tree. The tropical rainforest is characterized by dense vegetation and heavy rainfall, and the trees in these forests can grow to be very tall and have large, heavy branches and trunks. The buttress roots help to distribute the weight of the tree and prevent it from toppling over in the strong winds and heavy rain. They act like pillars, providing stability and support to the tree, especially in the shallow, nutrient-poor soils of the rainforest. Option 4 correctly explains the purpose of the buttress roots in relation to the mechanical load that the hardwoods put on the tree.