Isolation joints are primarily used to:

Isolation joints are primarily employed to separate structural elements that experience differences in loading and settlement. This is crucial in construction and civil engineering because different parts of a structure may move or settle differently due to various factors such as temperature changes, moisture variations, or uneven loading conditions. By incorporating isolation joints, engineers allow for independent movement of these elements, reducing the risk of stress concentrations and potential damage that could arise from rigid connections.

While all the other options touch on aspects of structural design, they do not capture the primary purpose of isolation joints as effectively. For instance, while controlling temperature changes is important, that function is more closely related to expansion joints, which are designed to accommodate thermal expansion and contraction rather than addressing differential movements due to varying loads or settlements. Similarly, enhancing shear strength and improving water drainage, while relevant in structural and drainage design, are not the primary concerns addressed by isolation joints. Therefore, the key role of these joints is to manage the differences in behavior between distinct structural components, making choice B the most accurate representation of their purpose.