Which of the following is NOT one of the three types of ignition systems?

The choice indicating "Cold-spark" is not recognized as one of the standard types of ignition systems used in many heating appliances, specifically in gas-fired systems. The primary types of ignition systems typically include hot-spark, direct-spark, and continuous ignition.

Hot-spark systems utilize an electric current to create a spark that ignites the fuel, which is a common method for ignition in many appliances. Direct-spark systems also operate on a similar principle but provide a direct ignition source that sparks directly at the burner. Continuous ignition systems offer a constant flame or heat source that maintains ignition without needing repeated sparking.

The term "Cold-spark" does not correspond to a widely acknowledged ignition system type, which differentiates it from the others listed. By understanding the standard types of ignition systems, it becomes clear why the term "Cold-spark" is the outlier in this context.