The correct answer is: A. distribution transformers.
Helical windings are used in distribution transformers because they are more efficient than other types of windings. They also have a lower profile, which makes them easier to install in tight spaces.
Distribution transformers are used to step down the voltage from high-voltage transmission lines to the lower voltages used in homes and businesses. They are typically located near the point where the transmission line enters a neighborhood or business district.
Power transformers are used to step up or step down the voltage in power transmission and distribution systems. They are typically much larger than distribution transformers and are located in substations or power plants.
Shell type transformers are a type of power transformer that has a shell-like core. The windings are placed around the core, and the core is then surrounded by a metal shell. This type of transformer is less common than core type transformers, but it has some advantages, such as being easier to manufacture and having a lower profile.
None of the above is not the correct answer because helical windings are used in distribution transformers.