How to populate a dynamically created select with options

You need to declare this ( current select-box) outside your ajax call and then access your select-box like below :

var selector = $(this); //declare this
  $.ajax({
      //..
    .done (function(responseJson1a){
        //other codes
      selector.closest('tr').find('.activityClass').html(activityOptionsNew);//add htmls
 })