Under what circumstance may more water be added to the concrete mixture?

The option regarding maintaining the slump while not exceeding the allowed water-cement ratio is correct because it addresses an important aspect of concrete mixing. The water-cement ratio is crucial in achieving the desired strength and durability of the finished concrete.

When adjustments to the water content are necessary to maintain a specific slump, which is a measure of the workability and fluidity of the concrete mix, it can be appropriate to add more water, provided that the additional water does not surpass the maximum allowable water-cement ratio. This ensures that while the concrete remains workable for placement and finishing, it retains sufficient strength and other performance characteristics once cured.

Maintaining the integrity of the water-cement ratio is essential for preventing issues such as reduced strength or increased permeability in the cured concrete. Therefore, the ability to add more water while adhering to this limit supports the overall quality and functionality of the mixed concrete.