Final answer:
The modal auxiliary in the sentence is 'must,' which is used to express an obligation or necessity, emphasizing the importance of being bold.
Step-by-step explanation:
In the sentence "We must be bold," the modal auxiliary is must. The function of this modal auxiliary is to express an obligation or a necessity. It indicates that being bold is not just a suggestion, but something that is required or deemed necessary in the situation being described.
Modals are used to indicate likelihood, ability, permission, and obligation, among other meanings. In this case, with the use of must, the speaker is stressing the importance of taking bold actions.