If the learner/student is offline:

  1. ....and proper doubt being explained by student/learner:
    1. You need to accept the doubt and can use the feature of Voice Notes to provide the best solution possible. 
    2. In case you think you have provided the best solution possible, you can mark the doubt as RESOLVED.
  2. ....and proper doubt not being explained by student/learner 
    1. You can mark the doubt as PENDING ON STUDENT if you need more information from the user. Avoid sharing messages like "Come Online", etc and try to mention proper explanations for marking Pending on Student.