Correct option is C
Back-to-Back dc-link:
A back-to-back connection is the direct connection of the output of one device to the input of a similar or related device.
A back-to-back connection for electric power transmission is a high-voltage direct-current (HVDC) system with both ends in the same switchyard.
This is used to couple asynchronously operated power grids or for connecting power grids of different frequencies where no DC transmission line is necessary.
The back-to-back dc-link reduces the overall conversion cost, improves the reliability of the DC system.
