Provide power to the 400 Hz main switchboards by operating in parallel or ______ configuration.

The appropriate term to complete the statement is "split plant" configuration. This refers to a setup where multiple power sources or generator sets operate together to supply power to the main switchboards in a coordinated manner, ensuring that there is adequate supply and redundancy.

In a split plant configuration, power can be distributed efficiently and can help in maintaining stability within the electrical system, particularly important for systems operating at 400 Hz often found in aviation and military applications. This arrangement allows for flexibility in load sharing and enhanced reliability, which is crucial when maintaining essential services.

Using other configurations like series or single line would not provide the necessary benefits of load distribution and redundancy. A synchronized configuration, while it implies working together, does not specifically indicate the division of resources that a split plant configuration does.