The current in the diode is the rate of flow of charge.
The charge on the capacitor = CV so the charge on it increases with the pd.
The current is equal to the rate of increase of this charge. (dQ/dt)
If the pd (and charge) increases uniformly, the current is constant.
I guess that is what the question intended. Your answer is definitely not correct.
If the pd line is slightly curved (such that its gradient gets less) then the current will decrease slightly.
I have no way of knowing, without the mark scheme, if this is what they intended.