If practical, the simplest alternative is (c), above, since it does not require any involvement of the HSA custodian (as does (a)) and does not incur any tax or potential 20% early-distribution penalty (as does (b)).
Note that an HSA custodian is not *required* to accept a return of mistaken distribution, although most do. The deadline is April 15 of the year following the year of distribution, however, since the HSA custodian has the option not to accept a return of mistaken distribution, you are probably better off returning the mistaken distribution before the end of the year. If the return of mistaken distribution is made and accepted after the custodian prepares the original Form 1099-SA that includes the mistaken distribution (but by the April 15 deadline), the custodian will have to issue a corrected Form 1099-SA.
