To define a default response to a question, use the "Default Data Value" field in the logic section of a question (for more information on using logic in CommCare visit the Common Logic and Calculations page). Set the Default Data Value for a question to be a case property of your choosing. Default Data Values also work for multi-select questions, if the value loaded into the multi-select question is a string of the Question ID's of the desired default responses, separated by spaces.
This functionality is useful, for example, if you want to have an Update Registration or Modify Registration form that defaults the response to a given question entered at registration. For example, if you have a registration form with a Multi-Select question to find out the occupation of a woman and during registration you check-marked three of the choices: farmer, seamstress, and hairdresser. The response to this question could be saved in the case property as 'farmer seamstress hairdresser'. If you default the question to that case property, 'farmer' 'seamstress' and 'hairdresser' will show up pre-checked.