The correct answer is D) overloading in all cylinders. If the cooling water temperature from all cylinders begins to rise above normal, it typically indicates that the engine is being overloaded, which causes the increased heat generation in the cylinders. This increased heat load on the cooling system results in the rise in cooling water temperature. The other options are incorrect because: A) insufficient fuel delivery would affect individual cylinders, not all of them; B) increased blow-by would also likely affect individual cylinders; and C) incomplete combustion may cause increased heat, but would not necessarily affect all cylinders uniformly.