During the positive half cycles of input ac, a $p-n$ junction diode is forward biased and hence it conducts.
While during the negative half cycles, the diode will be reverse biased and hence does not conduct.
The capacitor is charged to maximum potential difference.
Therefore, the potential difference across capacitor $C$ is equal to the peak value of the applied ac voltage, i.e..
$V = V _{\max }$
or, $V =\sqrt{2} V _{ rms }$
or, $V =\sqrt{2} \times 220 \,V =200 \sqrt{2}\, V$
