I think you mean "what" here. If they said "which" then they would have given you choices to pick from.
what is your line of work?
"I'm working in the health field." is a general response if you don't want to be specific about what you actually do. Be prepared though for a follow-up question asking you specifically what your job is if the person is persistent.
"I'm a doctor." is fine if it is true.
Student or Learner