Answer:
A. Wind depositing sand to build up sand dunes.
Step-by-step explanation:
A constructive force is one that builds up or adds material to the Earth's surface. In the case of wind depositing sand to build up sand dunes, it illustrates a natural process that acts as a constructive force by adding to and shaping the landscape.
Incorrect: B. A glacier weathering a mountainside - Glacial weathering typically involves the breaking down or erosion of rocks and mountainsides. It is more associated with destructive forces as glaciers carve and shape landscapes through erosion.
Incorrect: C. A river eroding soil during a flood - River erosion during a flood is a process that removes or wears away soil, contributing to the loss of material from the Earth's surface. This is more aligned with destructive forces as it involves the removal of material.
Incorrect: D. Acid rain breaking down the surface of a rock - Acid rain contributes to the breakdown of rocks, causing weathering and erosion. This is a destructive force as it involves the decomposition or breakdown of the surface of rocks.
In contrast, option A describes a process where wind is depositing sand, contributing to the building up and formation of sand dunes. This accumulation of material represents a constructive force, as it adds to and shapes the Earth's surface.