_______________ could cause an explosion if it is used to purge an air conditioning system while brazing.

Oxygen can cause an explosion if used to purge an air conditioning system while brazing because it supports combustion. When oxygen is introduced into a system being brazed, it can react with flammable materials or gases present within the system, leading to a rapid and possibly explosive reaction.

In contrast, inert gases like nitrogen and helium are safe for purging since they do not support combustion and help to prevent oxidation during the brazing process. Using inert gases minimizes the risk of chemical reactions that could lead to dangerous situations. Thus, utilizing oxygen in this context is incorrect and unsafe, making it the right choice for identifying a potential cause of an explosion during brazing.