a or b; c would be possible only in a jocular context where the pattern 'once a <period>' had been established:
'How often do you go - once a year? once a month? once a week?'
'Once a three days, actually'. [This is wrong, but could be used as a joke (and some linguists would object to my use of the word 'wrong').]
For (d), as the frequency is approximate ('roughly/around/about') you could say 'twice a week'.
Student or Learner