What is to throw someone's hat in the ring?

[ESL Student]

ESL/EFL Test "Talking through his hat", question 5

At last she's made her mind up and decided to throw her hat in the ......... and become a candidate.

(a) circle
(b) ring
(c) round
(d) middle

ESL/EFL Test "Talking through his hat", answer 5

At last she's made her mind up and decided to throw her hat in the ring and become a candidate.

Correct answer: (b) ring

Your answer was: correct
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Hi Alan,
What is to throw someone's hat in the ring?

[Mister Micawber]

.
To enter a contest or competition. Presumably there was at one time some sort of competition where one actually did this to indicate one's participation.
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Native English teacher at Mister Micawber's