Refer to figures.
Carbon monoxide (CO) is particularly dangerous during an engine fire because it is a colorless, odorless gas that can be highly toxic. It is produced from the incomplete combustion of fuel. Inhaling carbon monoxide can lead to poisoning, which can cause symptoms such as headache, dizziness, confusion, and even death in high concentrations. Its toxicity and the fact that it is not easily detected make it especially hazardous during and after an engine fire.

Nitrogen is an inert gas and does not typically pose a danger in the context of an engine fire. It is a major component of the atmosphere and is not toxic or reactive under normal conditions.

Moreover, while oxygen is essential for combustion and can support fire, it is not itself toxic. In high concentrations, it can exacerbate fires, but it does not pose a direct poisoning risk like carbon monoxide.

Finally, Carbon dioxide (CO₂) is used in fire extinguishers and is not toxic in normal concentrations. However, in very high concentrations, it can displace oxygen and cause suffocation, but this is less of a concern compared to carbon monoxide during an engine fire.

Therefore, "Carbon monoxide (CO)" is the correct answer.