What type of explosion is characterized by a flame front advancing at less than the speed of sound?

The correct choice is deflagration, which is characterized by a flame front advancing at a speed less than the speed of sound. This type of explosion involves a combustion process where the heat generated by the reaction propagates through the unreacted fuel mixture. Deflagrations are typically associated with materials that burn relatively slowly and can include scenarios such as a gas leak igniting in an enclosed space.

In contrast, detonation is characterized by a rapid combustion process that occurs at or above supersonic speeds, leading to a shock wave that compresses and ignites the unreacted material ahead of the flame front. Instantaneous explosion refers to a sudden release of energy that occurs without a controlled flame front propagation. Oxidation is a broader chemical reaction involving the combination of a substance with oxygen, which can lead to combustion but doesn't specifically describe the speed at which the reaction propagates.

In summary, deflagration is the only type among the options that correctly defines an explosion with a flame front that advances slower than sound, making it the appropriate answer.