Which of the following is NOT an admixture class?

The identification of "C. Filler" as the option that is not an admixture class is accurate because filler materials do not fit the standard classification of concrete admixtures utilized in the formulation of concrete.

Admixture classes are generally defined by their functional roles in enhancing specific properties of concrete. Water reducers, for instance, are employed to decrease the water-to-cement ratio while maintaining workability, leading to improved strength and durability. Chemical accelerators function to speed up the hydration process, which can be particularly advantageous in colder weather or when faster setting times are required. Hydration control admixtures play a role in managing the setting and curing of concrete, preventing premature hardening and allowing for better workability.

In contrast, a filler is often considered more of a component material rather than an admixture that modifies the properties of the concrete mix. Fillers, such as sand or other fine aggregates, are used to occupy space and may enhance workability or reduce the overall cost of concrete, but they do not belong to the standardized classes of admixtures designed to impart specific chemical functions to the mix.