Both are possible, but the meaning is different: sie = 3rd person singular: Sie ist Studentin? (talking about someone else) Sie = 2st person singular, polite address: Sie sind Studentin? (addressing the female student) The grammatical form and usage of the polite address is identical to the 3rd person plural.

Both are correct, but it depends on the meaning: Sie sind Studentin? You are a student? 您是学生，对吗？ Sie ist Studentin? She is a student? 她是学生， 对吗？ The question is a rhetorical one. The real question would be: Sind Sie Studentin? Are you student? 您是学生吗？ Ist sie Studentin? Is she a student? 她是学生吗？ The pronoun is not the same: Sie (you, 您), sie (she，她)!

"Sie ist Studentin" would be correct. Because, in this case, "sie" is a singular pronoun meaning "she" and singular nouns or pronouns in the 3rd person (he, she, it) will always use "ist." "Sind" would be used when the noun were plural. For example, by changing the singular noun "Studentin" to its plural form "Studentinnen," then you could use "sind:" "Sie sind Studentinnen." If you need more clarification or have any more questions, please let me know.