Which term describes someone who remains calm and composed, not easily upset or excited?

The term that best describes someone who remains calm and composed, not easily upset or excited, is imperturbable. This word specifically refers to a person’s ability to remain unflustered and serene in the face of adversity or chaos. An imperturbable individual demonstrates an exceptional level of emotional stability, allowing them to deal with stressful situations without losing their cool.

The other choices present contrasting meanings. Marginal typically refers to something that is not central or significant, often suggesting that it is on the outskirts of importance. Maverick describes a person who exhibits independence in thought and action, often defying norms or conventions. Mitigate means to make something less severe or intense, which does not relate to the emotional steadiness described in the question. Each of these terms has a distinct connotation that does not align with the calm and composed nature of an imperturbable person.