Which term would best describe an object that is resistant to being extinguished?

The term that best describes an object that is resistant to being extinguished is "incombustible." This term refers to materials that do not burn easily or cannot be burned at all. Incombustible materials can withstand high temperatures without catching fire, making them valuable in various contexts where fire resistance is essential, such as construction or fire safety.

In contrast, the other terms do not accurately convey the same meaning. For instance, "indelible" refers to something that is not able to be erased or removed, and is primarily used in the context of ink or marks rather than fire or combustion. "Inflammable" means that something can easily catch fire and burn, which is the opposite of being resistant to extinguishment. Lastly, "irreversible" describes a process that cannot be reversed or undone, which is not relevant to the context of fire resistance. Therefore, incombustible is the most appropriate choice for an object resistant to being extinguished.