What term signifies the action of lessening the seriousness or extent of something?

The term that signifies the action of lessening the seriousness or extent of something is "mitigate." This word is commonly used in contexts where reducing the severity or seriousness of a problem, condition, or situation is discussed. For instance, in legal, environmental, or health-related discussions, mitigating actions can lessen risks or adverse effects.

In contrast, the other options do not convey this meaning. "Ostracism" refers to exclusion or banishment from a society or group, which does not relate to the concept of reducing severity. "Sporadic" describes something occurring irregularly or infrequently, not implying any reduction in seriousness. "Monotonous" refers to something that is dull or repetitive, which is also unrelated to the concept of mitigation. Thus, "mitigate" is the appropriate term that captures the essence of lessening the seriousness or extent of a situation.