Which of the following best describes a preventative measure for shallow water blackout?

Limiting breath-holding time is indeed the best preventative measure for shallow water blackout. This condition can occur when a diver holds their breath for an extended period, particularly due to hyperventilation prior to submersion. Hyperventilation can reduce carbon dioxide (CO2) levels, delaying the body's natural urge to breathe, which can lead to loss of consciousness underwater before the diver feels a need to breathe.

By limiting breath-holding time, divers are less likely to experience decreased CO2 levels, allowing the body's natural safety mechanisms to work properly. This means divers are more aware of their need to surface for air, reducing the risk of a shallow water blackout.

The other choices do not provide effective preventative measures. Allowing full hyperventilation can increase the risk of blackout, while encouraging prolonged underwater stays directly contradicts the principle of limiting breath-holding. Ignoring breath urges would similarly heighten the risk, as it disregards the body's signals informing the diver when to surface for air.