The answer is D. Replace the toner cartridge.
Vertical lines on a laser printer output are usually caused by a faulty or empty toner cartridge, a dirty or damaged drum unit, or a clogged or obstructed laser glass. According to the web search results, some possible steps to troubleshoot and resolve this issue are:
* Check the toner level and replace the toner cartridge if it is low or empty1.
* Remove the toner cartridge and inspect the drum unit for any signs of damage, such as scratches, cracks, or toner buildup. Clean the drum unit with a soft cloth or replace it if it is damaged23.
* Locate the laser glass, which is a narrow strip of glass under the scanner lid or inside the printer. Clean the laser glass with a lint-free cloth moistened with water and remove any dust, dirt, or debris that might block the laser beam24.