This book is mine
It is I who called you
My is used when you have a noun following it (ex. my phone, my car) but mine will be used as a pronoun to replace the noun (ex. it is mine, the blanket was mine). So in this case, we would use mine instead of my.
In formal english it is I is the appropriate phrase as me would only be used if it was the object of a preposition (ex. at me, to me). However, it is me is more commonly used today in a more informal, casual manner. For the purpose if the assignment, the answer would be it is I.
