What marital status am I?

Lunar_Eclipse

Since the question asks for your marital status, I'd write 'unmarried' or 'never married' as opposed to single or living with partner, which don't technically answer the question.

The options are something like: unmarried, married, separated, divorced, widowed.

Alter_ego

Marital means concerning marriage. Your true status in that respect is unmarried.

Newly_retired

Absolutely right for this context. But be aware that as far as the DWP is concerned, for the purpose of benefits and tax credits, if you live together you are assessed as a household, whether married or living together.
Just for information.

jackyann

I would ask your manager / HR what the purpose of the question is.
Some organisations have fairly formal arrangements to allow time off to care for a 'partner' or bereavement (sorry!) and need to know who this is.