Think in terms of local solidification time rather than the growth-rate formula itself. When the cooling rate is high, the metal spends only a short time between the liquidus and solidus temperatures, so the dendrite network has no chance to coarsen or spread its arms further apart before the whole region is solid. A slow cooling rate does the opposite, giving atoms plenty of time to diffuse, so secondary arms coarsen and end up spaced further apart. So a shorter freezing window, which comes from faster cooling, always leaves behind a finer, more closely packed dendritic structure. This is exactly why thin, rapidly cooled sections show much smaller dendrite arm spacing than large, slowly cooled sand castings. Hence the correct choice is option (B).