Question: "Who was one of the pioneers using Hauber's salt in the United States?"

Context: "Phase change materials such as paraffin wax and Glauber's salt are another thermal storage media. These materials are inexpensive, readily available, and can deliver domestically useful temperatures (approximately 64 °C or 147 °F). The "Dover House" (in Dover, Massachusetts) was the first to use a Glauber's salt heating system, in 1948. Solar energy can also be stored at high temperatures using molten salts. Salts are an effective storage medium because they are low-cost, have a high specific heat capacity and can deliver heat at temperatures compatible with conventional power systems. The Solar Two used this method of energy storage, allowing it to store 1.44 terajoules (400,000 kWh) in its 68 cubic metres storage tank with an annual storage efficiency of about 99%."

Answer:
The "Dover House
The question is asking who was one of the pioneers using Hauber's salt in the United States. The context describes how Glauber’s Salt can store heat from solar energy, and it gives an example to support this: "The "Dover House" (in Dover, Massachusetts)". Therefore, we know that whoever first used Glauber’s Salt for thermal storage in the US was a pioneer user of this technology.
So based on these facts alone, you could conclude that The “Dover House” is our answer since they were mentioned as having been the first people to use Glauberger's salt for heating system back in 1948..