The phrase it is I is correct for formal writing. Traditionally, the use of I is appropriate when it follows a linking verb like is, was, or were. Linking verbs express a state of being rather than describing an action. They're usually paired with subject pronouns.
1. “She” is an object pronoun while “her” is a possessive pronoun. 2. “She” is used for the subject of the sentence while “her” is used for the object of the sentence.