Final answer:
The peptide precursor for oxytocin is neurophysin, which is a protein carrier, and it forms part of the structure necessary for the hormone's function.
Step-by-step explanation:
The peptide precursor for oxytocin is known as neurophysin, which is a protein carrier. It comprises a chain of amino acids that are essential for the hormone's structure and function. Specifically, oxytocin's structure is of a polypeptide chain, similar to vasopressin, another peptide hormone produced by the pituitary gland, but they differ in two amino acid residues at positions 3 and 8. Both hormones have structures that include cysteine residues forming a disulfide bond. When answering the question about the precursor for oxytocin, the correct choice is D) Neurophysin, a protein carrier.