What is the correct action when resistance is met while inserting a nasopharyngeal tube into a casualty's nostril?

When resistance is met while inserting a nasopharyngeal tube, the appropriate action is to remove the tube and insert it into the other nostril. This approach is based on the anatomy and physiology of the nasal passages; each nostril has a slightly different size and orientation, and inserting the tube into a different nostril can often facilitate easier passage without causing trauma.

Continuing to push the tube firmly can lead to injury to the nasal cavity or cause further complications, such as perforating the nasal septum. Using lubricant may help in some situations but does not address the core issue of anatomical resistance. Similarly, using a nasal spray may temporarily relieve discomfort, but it doesn't effectively solve the problem of physical blockage or resistance encountered during insertion. Therefore, switching to the other nostril is the most effective and safest method to ensure proper placement of the nasopharyngeal tube in the casualty.