Each return required under the regulations in this subpart shall, if signature is called for by the form or instructions relating to the return, be signed by (a) the individual, if the person required to make the return is an individual; (b) the president, vice president, or other principal officer, if the person required to make the return is a corporation; (c) a responsible and duly authorized member or officer having knowledge of its affairs, if the person required to make the return is a partnership or other unincorporated organization; or (d) the fiduciary, if the person required to make the return is a trust or estate. The return may be signed for the taxpayer by an agent who is duly authorized in accordance with § 31.6011(a)-7 to make such return.