The correct answer is C) if possible, open the delta-connections to avoid shunting the phase being tested. When testing a three-phase delta-connected winding for an open circuit using an ohmmeter, it is important to open the delta-connections to avoid shunting the phase being tested. This allows the ohmmeter to accurately measure the resistance of the individual winding without interference from the other phases. If the delta-connections remain intact, the ohmmeter reading may be affected by the parallel paths created by the other phases, leading to an inaccurate measurement. The other options are incorrect because: A) measuring the voltage across the open connections is not necessary for this test, B) testing the windings as parallel groups could introduce short circuits, and D) testing each phase with all connections intact would not provide the accurate individual winding resistance measurement required.