Which of these is a common source of ignition?

A common source of ignition is indeed power cords. Power cords can become a fire hazard due to various factors such as fraying, damage, or overload. They can generate heat when they carry excessive current, especially if they are not properly rated for the devices they are connected to. In some cases, cords can short-circuit, leading to sparks that can ignite nearby flammable materials.

In contrast, water sources are typically used to extinguish fires rather than ignite them, making them an unlikely source of ignition. Food products, while they can potentially combust under the right conditions (such as excessive heat), are not standard ignition sources. Medical supplies generally include items that are not prone to catching fire spontaneously and are also used in controlled environments where safety protocols mitigate such risks. Therefore, power cords stand out as a common ignition source due to their prevalence and potential for hazards related to improper use or maintenance.