Final answer:
A hormone with four amino acids is classified as a peptide hormone, which is water soluble and requires cell surface receptors to interact with target cells. Therefore, correct option is a.
Step-by-step explanation:
A newly discovered hormone that contains four amino acids linked together would most appropriately be classified as a peptide hormone.
Peptide hormones consist of chains of amino acids, with a length that can range from a few to over 200 amino acids. In contrast, steroid hormones are derived from lipids, specifically cholesterol, and amino acid-derived hormones are made from a single amino acid or its derivatives.
Since the hormone in question is comprised of four amino acids, it definitely falls into the peptide category. It is also important to note that peptide hormones are generally water soluble and cannot diffuse across plasma membranes, thus they need cell surface receptors to exert their effects on target cells.