How far should a service switch be from the burner?

The correct answer indicates that the service switch should be within arm's reach while observing the flame. This positioning is crucial for safety and convenience. It allows the technician or operator to easily turn off the burner if necessary, ensuring they can quickly respond to any unsafe conditions, such as an irregular flame or malfunction.

Being within arm's reach also facilitates monitoring of the flame without the need to move away from the burner, which can be particularly important during the initial startup or troubleshooting process. This practice aligns with safety standards in the industry, emphasizing the importance of having control over the equipment while still being able to actively observe its operation.

The other options, while they offer varying distances, do not adequately balance safety with the practical need for close observation during operation.