A p-n junction diode is forward-biased when the potential at the p-side (anode) is greater than the potential at the n-side (cathode), i.e., $V_p > V_n$. Conversely, if the n-side is at a higher potential ($V_n > V_p$), it is reverse-biased. By evaluating the voltage at both terminals of the diodes in the given figures (not visible here), we can deduce that the circuits (I) and (III) satisfy the condition for forward biasing.