Which of the following is a dangerous byproduct of incomplete combustion?

Carbon monoxide is a dangerous byproduct of incomplete combustion because it is a colorless, odorless gas that can be highly toxic. When a fuel such as natural gas or propane doesn't combust completely due to insufficient oxygen or incorrect burner settings, it can produce carbon monoxide instead of carbon dioxide, which is a normal byproduct of complete combustion.

Carbon monoxide binds to hemoglobin in red blood cells more effectively than oxygen, which can lead to reduced oxygen delivery throughout the body. Even at low levels of exposure, carbon monoxide can cause symptoms such as headaches, dizziness, confusion, and in severe cases, it can lead to unconsciousness or death.

The other options, while they may result from combustion processes, do not pose the same immediate health risks as carbon monoxide. For instance, hydrogen is not typically a byproduct of common fuel combustion in commercial food equipment, carbon dioxide is a normal byproduct and less harmful in typical concentrations, and water vapor, also a normal output of combustion, is not toxic. This makes carbon monoxide the significant concern in the context of gas safety.