Final answer:
Mouth care for clients on oxygen should be performed every 4 hours to prevent drying out of the mucous membranes and reduce the risk of infection.
Step-by-step explanation:
The frequency of mouth care for clients on oxygen should be every 4 hours, option (a). This is because oxygen therapy can dry out the mucous membranes in the mouth, increasing the risk for infection and discomfort. Thus, regular mouth care is important to maintain oral hygiene and comfort for these individuals. While the exact frequency might vary based on individual needs and institutional protocols, as a general rule, mouth hygiene should be performed more frequently in those receiving oxygen therapy.