What action should the Project Manager take regarding the remaining windows?

All requirements that have not been met are automatically carried over to the next increment.

Extend the time allocated to Timebox C, to allow time to complete these.

Insist that all available resources within the Solution Development Team are redirected to this work over the remaining two days.

Decompose the individual elements of work to be completed on the windows and reprioritise them to meet the Minimum Usable Subset.