Which word refers to a burden that is troubling or difficult to bear?

The term "onerous" specifically describes a task or responsibility that involves a significant burden or difficulty. It conveys the idea that something is not only heavy or challenging in a physical sense but also figuratively weighty, creating a sense of hardship for the person who must deal with it. This makes it particularly suitable for situations where a burden is troublesome or demanding, aligning perfectly with the question's context.

In contrast, "pernicious" refers to something that is harmful or insidious, but it does not specifically denote a burden that is difficult to bear. "Spurious" describes something that is false or not genuine, which is unrelated to the concept of burden. Lastly, "pedantic" relates to being overly concerned with minute details or rules, especially in teaching, and does not capture the essence of a troubling burden. Thus, "onerous" stands out as the most appropriate choice.