Which type of tank is capable of being removed for maintenance?

Bladder tanks are specifically designed to be removable for maintenance purposes. These tanks are typically made of flexible materials and are often installed within a frame or a cavity of an aircraft. Their design allows them to be easily accessed and removed, facilitating maintenance, repairs, or replacements without significant disassembly of surrounding components.

This feature is particularly advantageous, as it allows for quicker maintenance turnaround times and less impact on the overall aircraft operation. In contrast, main tanks and auxiliary tanks are generally fixed in place and require extensive work to access them if repairs are needed. External tanks, while removable, are typically used for additional fuel capacity and are not designed with maintenance access as a primary consideration. Thus, when considering tanks that allow for maintenance ease and flexibility, bladder tanks stand out as the correct answer.