code: def challenges(self):
          
          if self._challenges is None:
              self._challenges = ChallengeList(
                  self._version,
                  service_sid=self._solution['service_sid'],
                  identity=self._solution['identity'],
                  factor_sid=self._solution['sid'],
              )
          return self._challenges
docs: Access the challenges
  
          :returns: twilio.rest.authy.v1.service.entity.factor.challenge.ChallengeList
          :rtype: twilio.rest.authy.v1.service.entity.factor.challenge.ChallengeList