Which of the following components is sometimes placed longitudinally in the container to support plywood flooring?

The hat section center spacer is designed to be placed longitudinally within the container to provide additional support for plywood flooring. This component enhances the structural integrity of the flooring by distributing loads evenly across the space, thereby preventing sagging or damage that can occur due to the weight of cargo. Its shape and positioning are particularly effective for maintaining the level of the floor and ensuring that excess stress does not concentrate in any one area, which can lead to deformation or failure over time.

Other components mentioned, such as the transverse bow and threshold plate, serve different functions in container construction and do not provide the specific support needed for plywood flooring in a longitudinal orientation. The gusset, while it may contribute to overall structural strength, is not primarily used for the purpose of supporting flooring directly. Therefore, the hat section center spacer is the correct choice as it directly correlates with enhancing the support for the plywood flooring in containers.