The statement that best describes an informal argument for the formula for the volume of a cylinder is:
C: a cylinder is a stack of h circles, each with an area of pi r^2 and a thickness of one unit, so its volume formula is V= pi r^2 h.
This explanation captures the essence of how the volume of a cylinder might be thought of informally as stacking circles (which are the bases of the cylinder) over the height of the cylinder, leading to the formula \( V = \pi r^2 h \).