Which of the following hand signals requires a response from others?

The hand signal for "Okay" is crucial in the context of communication among divers. This signal is used to verify that all divers are in good condition and that there are no issues that need to be addressed. When one diver gives the "Okay" signal, it prompts others to respond with their own "Okay" signal, confirming that they too are safe and ready to continue the dive. It's an important affirmation of safety and mutual awareness in an underwater environment where verbal communication is not possible.

In contrast, signals such as "Caution," "Prepare to ascend," and "Hold" are typically used to convey specific instructions or alerts, and while they are important for communication, they do not necessarily require a reciprocal acknowledgment from others in the same way that the "Okay" signal does. The response to these signals might depend on the context, but they do not inherently entail a check on the status of the other divers, which is the critical aspect of the "Okay" signal.