Which term would best describe a person's firmness of character?

The term that best describes a person's firmness of character is "mettle." Mettle refers to the ability to cope with difficulties in a spirited and resilient manner, showcasing strength of character and determination. This term conveys not just an inner strength, but also the capacity to persevere in the face of challenges, reflecting the essence of firmness as an attribute.

In this context, the other options do not convey the same meaning. "Minatory" relates to threatening or menacing actions, which do not pertain to character strength. "Mendicant" describes a person who begs for charity, often reflecting dependence rather than firmness. "Jejune" signifies something that is simplistic or superficial, lacking depth or maturity, which stands in contrast to the notion of having a strong, well-developed character. Thus, "mettle" is the most suitable choice to depict firmness of character.