The most common gas used in dentistry is:

The correct answer is nitrous oxide, commonly referred to as "laughing gas." This gas is frequently used in dentistry because of its sedative properties, which help to calm patients during dental procedures. Nitrous oxide works quickly and is easily controllable, providing anesthesia that is safe for a wide range of patients, including children and those with anxiety.

While oxygen is essential for patient safety and is used during procedures, it is not primarily used as a sedative or anesthesia. Carbon dioxide is not utilized in dental practices for sedation and has no anesthetic properties. The term "E cylinder" refers to a specific type of gas cylinder used to store gases like oxygen or nitrous oxide, but it is not a gas itself. Hence, the focus should be on nitrous oxide as the primary anesthetic gas commonly administered in dental settings.