The correct answer is B) replacing the plunger and the barrel. The close tolerances used in diesel engine fuel oil pumps require a precise fit between the plunger and the barrel. When the plunger becomes worn, it no longer fits the barrel properly, and simply replacing the plunger alone will not restore the necessary tight fit. To ensure proper operation and prevent fuel leaks or other issues, both the plunger and the barrel must be replaced as a matched set. The other answer choices are incorrect because grinding the spare plunger (A) would not address the worn barrel, and highly polishing both the plunger and barrel (C) would not restore the necessary tight fit if the components are worn. Replacing the plunger only (D) would not fix the problem, as the worn barrel would still need to be replaced.