Final answer:
The maximum flow rate to use when applying supplementary oxygen via nasal cannula is 6L/min.
Step-by-step explanation:
When applying supplementary oxygen via nasal cannula, it is important not to exceed 6L/min (option b). This is because exceeding this flow rate can lead to discomfort and dryness of the nasal passages, as well as interfere with the natural breathing pattern. Higher flow rates may require the use of a different oxygen delivery system, such as a high-flow nasal cannula or a face mask.