update_form_by_spreadsheet.gs

// with thanks to http://wafflebytes.blogspot.com/2018/12/google-script-revisiting-making-form.html and http://wafflebytes.blogspot.com/2016/10/google-script-create-drop-down-list.html
// that script has some minor errors, notably asMultipleChoiceItem should be asCheckboxItem 
// to find the question ID open the form in preview, and inspect element on the question. Scroll up the divs and you should see something like data-params='%.@.[ - the ID should be in that (and then the question text)

// In the googlesheet you can use the following formula to pull from the relevant column in FormResponses1 AND prepopulate this with your own responses (E2:E20) in GSOptions sheet
// =unique(query({GSOptions!E2:E20;'FormResponses1'!G2:G}))

// In the googlesheet if you want to concatenate two fields for question options you can do this
// =transpose(query(transpose(UNIQUE(query('Form Responses 1'!A2:C, "select C,A order by C asc",1))),,COLUMNS(UNIQUE(query('Form Responses 1'!A2:C, "select C,A order by C asc",1)))))
// w/ thanks https://stackoverflow.com/questions/42571114/how-to-use-concat-in-query

function updateForm(){
  // call your form and connect to the multiple choice item
  // find the google form ID in edit mode between /forms/d/ and /edit
  var form = FormApp.openById("your-identifier-here");
   
  // identify the sheet where the data resides needed to populate the multiple choice
  var ss = SpreadsheetApp.getActive();
  var names = ss.getSheetByName("GSOptions");

  // get the question IDs you want to update/set
  // if you're doing this for multiple questions you could declare multiple things here, just keep track of them
  var namesList = form.getItemById("1234567").asCheckboxItem();
  var namesList2 = form.getItemById("7654321").asCheckboxItem();
  var nameSingle = form.getItemById("123654").asMultipleChoiceItem();

  // get the data from the sheet (identified above by name)
  // grab the values in the first column of the sheet - use 2 in first position (i.e., (2, x) to skip header row.
  // change the 1 (i.e., (x, 1), to the column you want (2 for column B, 3 for column C, etc).
  var namesValues = names.getRange(2, 1, names.getMaxRows() - 1).getValues();
  var namesValues2 = names.getRange(2, 2, names.getMaxRows() - 1).getValues();
  var namesValues3 = names.getRange(2, 3, names.getMaxRows() - 1).getValues();

  var principles = [];
  var context = [];
  var cases = [];

  ///////////////////////////////////////////////
  // convert the arrays ignoring empty cells
  ///////////////////////////////////////////////

  // First for principles 
  for(var i = 0; i < namesValues.length; i++)    
    if(namesValues[i][0] != "")
      principles[i] = namesValues[i][0];
  // populate the drop-down with the array data
  namesList.setChoiceValues(principles);

 // Second for context
  for(var i = 0; i < namesValues2.length; i++)    
    if(namesValues2[i][0] != "")
      context[i] = namesValues2[i][0];
  // populate the drop-down with the array data
  namesList2.setChoiceValues(context);

   // Third for cases
  for(var i = 0; i < namesValues3.length; i++)    
    if(namesValues3[i][0] != "")
      cases[i] = namesValues3[i][0];
  // populate the drop-down with the array data
  nameSingle.setChoiceValues(cases);

}