Which adjective describes a situation that cannot be defended or maintained?

The adjective that describes a situation that cannot be defended or maintained is "untenable." This term is often used in contexts such as academic debates, logical arguments, and discussions about policies or positions that lack support or justification. For example, if a theory or argument is called untenable, it indicates that the reasoning behind it is flawed to the point where it cannot be logically held or sustained under scrutiny.

The choice of "untenable" captures the essence of being indefensible very precisely, whereas the other options refer to different concepts. "Protracted" means prolonged or drawn out, often referring to a situation that lasts longer than expected. "Onerous" describes something that is burdensome or involving a lot of effort and difficulty, while "adversarial" pertains to opposition or conflict but does not imply a lack of defendability. Therefore, in the context of the question, "untenable" is the only word that aligns with the inability to defend or maintain a situation.