Cavitation occurs when the absolute pressure is less than or equal to the vapor pressure (pv), so that the water is allowed to "boil".
For water temperature of 50 oC, the vapor pressure is
12.33 kPa. Since the operating pressure of the pump on the inlet side is 6 kPa,
which is less than the pv, cavitation will occur.
On the other hand, for water temperature of 20 oC, the vapor
pressure is 2.338 kPa. The operating pressure of the pump on the
inlet side is
now greater than the pv, hence cavitation will not occur.