Choice (B) is correct. If these terms were inserted into the sentence, the sentence would read, “While a 'rock' is usually defined as an aggregate, or combination, of one or more minerals, geologists often extend the definition to include such materials as clay, loose sand, and certain limestones.” An “aggregate” is the sum total of a combination of things, so a “rock” is an aggregate of one or more minerals. In addition to minerals, geologists feel confident including other materials such as “clay, loose sand, and certain limestones” in their definition of a rock; they “extend,” or broaden, the definition.