Form 8615: what should I select for my parent's US tax filing status, if my parents don't file US taxes? (Options:married filing separately, single, head of household)

My parents are not US citizens and don't reside in the US, and thus they don't file taxes in the US.

I am filling out 8615 because I am a US resident for tax purposes and received a stipend income last year.

I am unable to complete Form 8615 without selecting my parent's US tax filing status on TurboTax and am only given a multiple choice with the following options: married filing separately, single, head of household.