The correct answer is: A. Bushings
Bushings are the parts of a transformer that are visible from outside. They are used to insulate the high-voltage windings from the low-voltage windings. Bushings are typically made of porcelain or ceramic, and they have a metal sleeve that fits over the end of the winding. The sleeve is connected to the transformer’s terminals, and the bushing provides a safe and reliable connection between the windings and the outside world.
The other options are incorrect because they are not visible from outside the transformer. The core is the central part of the transformer, and it is made of a magnetic material such as iron or steel. The primary winding is the winding that is connected to the power source, and the secondary winding is the winding that is connected to the load. Both the core and the windings are enclosed in a metal case, and the bushings are the only parts of the transformer that are visible from outside.