What is a primary advantage of a "pony bottle"?

A pony bottle is a small, independent scuba tank that serves as an additional air source separate from the primary tank. This setup is particularly advantageous for divers in case of an emergency, such as a malfunction with their main tank or regulator. Having a dedicated source of air can provide peace of mind and an essential safety measure, allowing a diver to manage situations where they may need to make a controlled ascent or swim back to safety.

This secondary air source can be crucial in scenarios where a diver encounters an unexpected air supply issue, effectively extending their dive time or ensuring their safe return to the surface. While the other options touch on various aspects of diving, they do not address the safety and emergency preparedness that having a pony bottle provides.