The modal auxiliary in the given sentence will be used as, She must have failed the exam. She was crying all her eyes out.
<h3>What is modal auxiliary?</h3>
Modal auxiliary are the verbs that are used to form the base form and to accompany another verb.
Because Modal auxiliary are the verbs that are used to form the base form and to accompany another verb. Modal auxiliary are denoted by the moods and feeling that is expressed in the sentence. Modal auxiliary are the words like, used to, must, shall, should, could. So, in the given sentence the modal auxiliary that is should be use is 'must' as it denotes the conclusion, logical or firm necessity.
Hence the correct answer is, Must.
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