Which of the following is a potential issue caused by manganese in water systems?

Manganese in water systems is known to cause black stains on fixtures, which is why this choice is the correct answer. When manganese is present in high concentrations, it can precipitate out of the water as it oxidizes, creating dark deposits that can cling to plumbing fixtures, laundry, and dishes. This staining can be particularly problematic in home settings and can also be challenging to remove from surfaces.

The presence of manganese typically does not lead to an increase in pH; instead, it is often associated with conditions that could even contribute to a lower pH in some cases. Additionally, manganese does not generally improve the taste of water; rather, it can impart an undesirable, metallic or earthy flavor when present in significant amounts. Finally, manganese itself does not cause decreased conductivity in water; in fact, the presence of dissolved minerals, including manganese, can often contribute to increased conductivity, indicating more ions in the water.