You're correct; I misread it for some reason as a position versus time graph. As far as 'speed increasing negatively' is concerneed, that's a perfectly valid way of specifying velocity.
How convinced are we that the answer is a.?
The graph basically says the following:
First linear slope: The speed of the bar magnet increases positively for a period of time
Nearing the crest: The bar magnet starts slowing down in the positive direction
Top of crest: The speed of the bar...