A packaged watertube boiler having two mud drums is the:

The configuration of a packaged watertube boiler with two mud drums describes an "A" type design. This type of boiler is characterized by its arrangement where the steam and water flows in a vertical manner, making it conducive for efficient heat transfer and steam production. The mud drums serve the purpose of collecting the sediment that would naturally settle in the boiler's water, ensuring a clean operating environment and enhancing boiler efficiency.

In contrast, the other options refer to different types of watertube boiler configurations. The three drum bent-tube type typically has additional drums and a more complex arrangement, while the "O" and "D" types are defined by different geometric arrangements and functionality. Each configuration serves specific operational needs and design requirements, but for a packaged watertube boiler with two mud drums, the "A" type is the correct classification.