Reactance to varying DC
A capacitor's reactance depends only on the value of capacitance and the frequency of the signal presented to it. Your example (b) has the same frequency as example (a), with the addition of a DC component, which has zero frequency. The long term effect of the DC component is to charge the capacitor to 5 volts. After that its reactance would be the same as in example (a). This charging takes place exponentially with time constant RC, so that as a practical matter after 4 or 5 time constants, equilibrium is reached.