Which term describes something that is impossible to tolerate?

The term that describes something that is impossible to tolerate is "intolerable." This word conveys the idea that a certain situation, behavior, or condition is beyond what one can accept or endure. It emphasizes an extreme level of discomfort or aversion, suggesting that the burden or situation is so severe that one cannot bear it any longer.

In contrast, the other terms do not carry the same meaning: "intrepid" refers to someone who is fearless or adventurous, "intractable" describes a problem that is difficult to manage or control, and "insular" relates to being isolated or detached, often referring to a mindset or perspective that is narrow and lacking openness to outside ideas. Therefore, "intolerable" is the word that best captures the essence of something that cannot be tolerated.