Which of these words could be used to describe a very weak argument?

The term "tenuous" is aptly used to describe a very weak argument because it literally means something that lacks a strong foundation or is insubstantial. When applied to an argument, "tenuous" conveys that the reasoning or evidence offered is fragile or only loosely connected, thus making the argument itself unreliable. This term often evokes the idea of something not being firmly established or likely to collapse under scrutiny, much like a weak argument that fails to hold up under critical examination.

Other choices have different meanings: "myriad" refers to an indefinitely large number and does not pertain to the strength of an argument. "Histrionic" describes overly dramatic or theatrical behavior, which may not accurately represent the nature of an argument's validity. "Obfuscates," meaning to make something unclear or confusing, relates more to the clarity of presentation rather than the inherent strength or weakness of the argument itself. Therefore, "tenuous" is the most fitting descriptor for a weak argument.