Answer:
I
Step-by-step explanation:
"I" is the simple subject of the sentence. A subject refers to the part of the sentence that performs the action described by the verb. In this case, the person that performs the action of "having" is "I." The sentence appears to be unclear because it is not ordered in the usual way. If we were to rearrange it, the subject would become clearer:
"I must have this book for my report."