The correct answer is A) replacing the plunger and the barrel. In diesel engine fuel oil pumps, the plunger and barrel are designed to work together with very close tolerances. When the plunger becomes worn, simply replacing the plunger is not sufficient, as the mating barrel will also be worn and cannot maintain the necessary tight fit with a new plunger. Replacing both the plunger and the barrel is required to restore the proper operation and sealing of the fuel oil pump. The other options are incorrect because: B) highly polishing both the plunger and barrel would not address the wear issues, C) grinding the spare plunger to the barrel is not a proper repair procedure, and D) replacing the plunger only would not resolve the problem caused by the worn barrel.