Final answer:
Breathing system pressure and leak testing should be done prior to each case and anytime the circuit is changed to ensure patient safety and equipment integrity.
Step-by-step explanation:
The breathing system pressure and leak testing should be performed prior to the start of each case (Option B) and anytime the circuit is changed (Option C). It is imperative for patient safety that the equipment is checked to ensure there are no leaks or faults. Although checking once per day (Option A) may be part of routine checks, it is essential that tests are conducted more frequently, as required by the specific clinical situation and whenever the integrity of the breathing system could have been compromised.