The fastest moving marble would be C.
because, A and D wouldnt work because the up-slopes cause friction that prevent the marble from going over them. So B and C are left. Looking at C first, at the start of the slope its low, causing little friction, so it falls faster. But after this steep slope it rises a bit, this new slope angle looks very similar to B, so without equations or numbers we can assume they are close enough to be considered the same slope. Marble C would fall faster because it has greater velocity already from the beginning of the slope, so Marble C reaches the bottom quicker.