Choosing a correct diode
(its like deja-vu all over again)

The reason for selecting a diode with certain parameters is as follows:

Reverse Voltage:  Select to be above the highest possible reverse voltage that might be seen across the device.  (When the diode is off.)

Forward Current:  Select to be above the highest possible current that might be seen flowing through device. (When the diode is on.)

Forward Voltage:  Select if the small voltage drop in the forward direction needs to be at or below a certain value. (When the diode is on.)

There are several other diode parameters but they are usually less important, especially in simple DC circuits.

For the circuit above:
Reverse Voltage:
Calculate the highest voltage that might be seen if the line (between c &amp; e) had the diode installed. Calculate the voltage in the reverse direction of the diode current.  (In other words put the diode in the schematic and do an analysis.)  In your last inquiry the question included the possibility of the other supplies failing, so do the analysis with that in mind).  The analysis will give an idea of the Reverse voltage needed.  You would normally select a diode with a reverse voltage above the calculation including some margin of safety (if that is given).  Diodes often come in certain reverse voltage values (Sort of like resistors that come in standard values)

Forward Current:
Same as above but calculate the highest possible current that might be seen passing through the diode in the forward direction.

Forward Voltage:
If the requirement is to have a certain known value, or a minimum value, for the forward voltage, then select the diode with that parameter. (For example, a silicone diode has about 0.7v forward voltage when on.)  Note - this parameter can be current dependent.

A Zener diode would only be used if you wanted the diode to pass current in the reverse direction at a known voltage.  These are normally used only if you want a fixed voltage at some point in the circuit.  For example, these can be used to create a voltage reference.