Which of the following enhances ground resistance effectively?

Copper sulfate is effective in enhancing ground resistance due to its chemical properties. When mixed with soil, copper sulfate helps to lower the electrical resistivity of the ground. The presence of copper ions improves conductivity, facilitating a better connection to the earth, which is crucial for grounding systems. This enhanced conductivity allows for better dissipation of electrical faults or charges, thus improving safety and system reliability.

In grounding applications, the goal is often to achieve a low resistance path to the ground. Copper sulfate achieves this by altering the soil's mineral composition, effectively enabling better conduction of electrical currents. This is particularly valuable in areas where soil conditions might otherwise lead to high resistance, which can compromise the effectiveness of grounding systems.

Other options such as HTML solution, tap water, and sand may not provide the same level of effectiveness in enhancing ground resistance. While tap water can conduct electricity, its impact is limited and temporary, as soil conditions can vary widely, affecting retention. Sand, on the other hand, tends to have high resistance, which does not aid in improving grounding performance. Thus, copper sulfate stands out as the most effective choice for enhancing ground resistance in grounding applications.