What is the voltage at the cathode of the upper diode if the other two diodes are off (i.e. open circuit) ?
Here's how to reason to the correct answer.
(1) If the other two diodes are off, the current through the upper diode is zero
(2) If the current through the upper diode is zero, the voltage across the upper diode is zero or negative.
(3) The voltage across the diode is the difference of the anode voltage and cathode voltage:
$$V_{diode} = V_{anode} - V_{cathode}$$
From (1), (2), and (3), it follows that the voltage at the cathode of the upper diode is 16V (or more).
But this is inconsistent with the other two diodes being off; if the cathode of the upper diode is 16V (or more), the voltage across the other two diodes is positive since their cathodes are at 12V (there is no current through the resistor).
Since we've reached a contradiction, we know that the other two diodes must in fact be on rather than off.