To determine the correct response, it would be important to know more about the specific pottery pieces in question, such as their historical context and stylistic characteristics. Generally, pottery can showcase various attributes including traditional techniques and innovative designs.
If the emphasis is on creativity and breaking from norms, the appropriate answer would be "experimentation with form that was not standard." If the focus is on adhering to established practices, then "standard usage of materials and design" might be correct.
Based on the available options, if we are discussing pottery that is notable for its uniqueness and creativity, then:
experimentation with form that was not standard
would likely be the most important factor present in the creation of the pottery pieces.