select.html

<form action="">
  <select class="target">
    <option value="option1" selected="selected">Option 1</option>
    <option value="option2">Option 2</option>
  </select>
  <input type="text" id="txt" name="" value="">
</form>

<script type="text/javascript">
$( ".target" ).change(function() {
  var str = "";
  $( "select option:selected" ).each(function() {
    str += $( this ).text() + " ";
  });
  $( "#txt" ).val( str );
});
</script>