The correct answer is B. duplexer.
A duplexer is a device that allows you to print on both sides of a sheet of paper. It does this by flipping the paper over after it has been printed on one side. This can save you paper and ink, and it can also be more environmentally friendly.
A fuser is a device that fuses toner to paper in a printer. It does this by heating the toner to a high temperature and then pressing it onto the paper.
A toner cartridge is a device that contains the toner that is used in a printer. Toner is a powder that is used to create images on paper.
A paper-swapping unit is a device that allows you to swap out paper trays in a printer. This can be useful if you need to print on different types of paper, such as letterhead or envelopes.
E is not the correct answer because it is not a device that allows you to print on both sides of a printer.