Final answer:
Direct materials would be classified as a variable cost in the cost hierarchy.
Step-by-step explanation:
In the cost hierarchy, direct materials would be classified as a variable cost. Variable costs are costs that change proportionately with the level of production. As the production of one style of workboots increases, the cost of direct materials will also increase. This is because the direct materials, such as leather or steel, are directly used in the manufacturing process of the boots.