Magnetic flux and EMF induction

The EMF is generated by a change in magnetic flux. That means a nonzero gradient on the flux vs time curve. At t=0, the flux is increasing (i.e. it was negative for t<0), which is already generating some EMF, which is why the EMF is positive at t=0. This is consistent with B but not A.

To check your understanding, try to work out what flux v time curve corresponds to A. It's pretty similar but there's a subtle difference around t=0.