Which term signifies something that involves hardship or difficulty?

The term that signifies something involving hardship or difficulty is "onerous." This word is often used to describe tasks, responsibilities, or obligations that are burdensome and require significant effort or hardship to fulfill. For instance, a job that demands excessive hours or complex responsibilities can be deemed onerous, as it imposes a heavy burden on the individual.

In contrast, "untenable" refers to a position or argument that cannot be defended or is unsustainable rather than implying hardship. "Protracted," on the other hand, describes something that is prolonged in time but does not inherently convey difficulty or hardship. "Ephemeral" means something that is short-lived or temporary, which does not relate to hardship or difficulty at all. Thus, "onerous" clearly captures the sense of challenging and burdensome conditions, making it the appropriate choice.