According to an idiom, when ever individual prove the strength of his/her character, then he/she has shown mettle.
Mettle can be regarded as person's ability to respond to difficulties; spirit and resilience.
It is an inbuilt vigor as well as strength of spirit or temperament.
Mettle helps to relate with all challenges by showing our strength and helps in overcoming them.
Therefore, the correct term is mettle.
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