What is the best term for a situation that is close to a lower limit or lower class?

The term that best describes a situation that is close to a lower limit or lower class is "marginal." When something is referred to as marginal, it indicates that it exists on the edge or periphery of what is considered normal or acceptable, often relating to low status, desirability, or quality. In contexts such as economics or social classes, marginal can refer to individuals or groups who exist at the limits of societal standards, often experiencing reduced resources or opportunities.

In contrast, the other options refer to different concepts that do not fit the definition provided. "Imperturbable" describes someone who is unable to be upset or excited; it has no relation to limits or class. "Mitigate" means to make something less severe, serious, or painful, which does not pertain to being near a lower limit. "Sporadic" refers to occurrences that are irregular or infrequent, which also does not connect to the idea of being close to a lower limit or lower class. Thus, "marginal" is the most appropriate choice in this context.