How can cave divers validate a signal underwater?

Cave divers can validate a signal underwater through a series of okay signals on an object because this method provides a clear and visible form of communication that can be easily recognized and understood among divers. Given the low visibility often encountered in cave diving, using a physical object to correlate signals enhances clarity and ensures that communicating divers understand each other's intentions and condition.

The use of physical signals, like tapping or gesturing towards an object while displaying an okay sign, takes advantage of the limited but effective means of non-verbal communication available in a submerged environment. This is particularly useful in situations where verbal communication is impossible, allowing divers to confirm that everything is alright or to communicate specific messages without the need for speech.

The other options don't provide the level of clarity or practicality needed underwater. Changing tank pressure does not convey specific signals to other divers and is more related to gauge readings. Using a whistle is not effective in a submerged environment, as sound waves travel differently underwater and might not be audible at distances. Similarly, writing in a dive log is impractical while diving and does not facilitate real-time communication.