Final answer:
The correct singular, third-person, conjugation of the verb to be in the future perfect tense is 'She will have been'.
Step-by-step explanation:
When selecting the correct singular, third-person, conjugation of the verb to be in the future perfect tense, the correct choice is A. She will have been. Options B, C, and D do not fit this criterion because they either do not represent singular third-person conjugation in the future perfect tense or are not in the future perfect tense at all. The future perfect tense describes an action that will have been completed by a certain point in the future.