Which adjustment is appropriate for an unworkable concrete mix that cannot be pumped?

Using high-range water-reducing admixtures is an effective adjustment for an unworkable concrete mix that cannot be pumped. These admixtures enhance the workability of the concrete by reducing the amount of water needed for a specific level of workability, allowing for a smoother mix without compromising strength. This makes the concrete more fluid and easier to pump, even with a lower water-cement ratio, which is crucial for maintaining the durability and strength of the concrete.

In contrast, options such as adding coarse aggregate might exacerbate the problem by increasing the mix's viscosity and reducing workability. Increasing the water-cement ratio can lead to a weaker concrete mixture and may still not resolve the pumpability issue. Likewise, reducing the cement content could compromise the concrete's strength and overall performance, worsening the original issue. Thus, using high-range water-reducing admixtures is the most suitable choice to improve pumpability and maintain structural integrity.