What type of boiler was commonly used around the year 1720?

The Haycock boiler is the correct choice as it was one of the earliest types of steam boilers used during the early 18th century, specifically around the year 1720. It is characterized by its simple design, consisting of a cylindrical firebox with a dome-shaped top. The Haycock boiler was designed to generate steam by using the combustion of fuel, usually coal or wood, beneath a water-filled chamber.

During this period, steam power was becoming increasingly important for various industrial applications, and the Haycock boiler played a significant role in the development of steam technology. This boiler's design allowed for larger steam generation compared to earlier boiling devices, making it suitable for tasks such as driving machinery and powering early steam engines.

The other types of boilers mentioned, such as the locomotive boiler, fire tube boiler, and water tube boiler, were developed later. The locomotive boiler, for example, became prominent with the advent of rail travel in the 19th century, while fire tube and water tube boilers evolved as engineering and materials improved, leading to advancements in boiler technology. Thus, the Haycock boiler represents the technology of its time and laid the groundwork for future developments in steam boilers.