Mass concrete is best defined as:

The definition of mass concrete is accurately described by the choice referring to concrete containing heavy aggregates. Mass concrete generally refers to large volumes of concrete that are placed in one operation or in a series of operations, often used in the construction of structures like dams, foundations, or large walls. The significant aspect of mass concrete is its ability to resist thermal cracking, which can occur due to heat generated from the hydration of cement. This is where heavy aggregates, which have a higher density and thermal mass, can help manage that heat build-up.

The other options do not capture the essence of what mass concrete entails. For example, concrete with minimal weight does not pertain to mass concrete, as mass concrete focuses on density and volume rather than minimizing weight. Similarly, concrete that has been molded prior to use refers more to the shaping process rather than the material properties that define mass concrete. Lastly, while some concrete mixes can be formulated to resist high temperatures, this characteristic is not exclusive or essential to mass concrete; thus, it does not define the concept. Understanding mass concrete's reliance on volume and thermal characteristics helps clarify why the choice concerning heavy aggregates is the most fitting definition.