For credit applications in which a lender may legal ask for each applicant's marital status, I thought the choices were limited to 1) Married, 2) Unmarried or 3) Separated.

Are we allowed to place on our online application these choices too? 4) divorced, 5) widowed, or 6) in a registered partnership

No:

1002.5(d) If an application is for other than individual unsecured credit, a creditor may inquire about the applicant's marital status, but shall use only the terms married, unmarried, and separated. A creditor may explain that the category unmarried includes single, divorced, and widowed persons.