Which of the following gases can be detected by a four gas meter?

A four gas meter is designed to measure specific gases that are commonly encountered in hazardous environments, particularly those related to pollution and industrial settings. Carbon monoxide is one of the key gases that the four gas meter can detect because it is a toxic gas produced by incomplete combustion of fossil fuels and is a significant concern for worker safety. The meter typically measures carbon monoxide concentrations to ensure that levels remain within safe limits.

While carbon dioxide, nitrogen, and ozone are important in various contexts, the standard four gas meter is not typically used to measure nitrogen, as it is a benign and non-toxic gas that constitutes a large part of the atmosphere. Carbon dioxide can be monitored in certain specialized situations, but it is not one of the primary gases detected by most standard four gas meters. Ozone is also not a standard measurement for these devices, as it generally requires different monitoring equipment due to its distinct chemical properties and health effects.

Thus, carbon monoxide stands out as the main gas among the options provided that will accurately be detected by a typical four gas meter.