The correct answer is: I have died.
A euphemisms is a word or a phrase that is substituted for a word or phrase that is considered to be too blunt or too straightforward when referring to something disturbing, sad, upsetting etc.
'I have been stilled' is a much gentler way of saying 'I have died'. It sounds less direct and less brutal.
This is a phrase from Thomas Hardy's poem 'Afterwards'.