The correct answer is A) Type K. Type K copper tubing has the thickest wall thickness among the three types (K, L, and M) for a given nominal size. This is because Type K is designed for the highest working pressure applications, while Type M has the thinnest walls and is intended for lower pressure uses. The wall thickness increases from Type M to Type L to Type K to accommodate higher pressure requirements. The other options are incorrect because Type L and Type M have progressively thinner wall thicknesses compared to Type K for the same nominal size, and the wall thicknesses are not identical across the three types.