view_tabel.php

<div class="row">
            <div class="col s3">
                <select name="" id="source">
                    <option value="" selected="" disabled="">Select Source</option>
                    <?php foreach ($source as $key):?>
                    <option value="<?php echo $key->source; ?>"><?php echo $key->source; ?></option>
                    <?php endforeach; ?>
                </select>
            </div>
            <div class="col s3">
                <select name="" id="scheme">
                    <option value="" selected="" disabled="">Select Scheme</option>

                </select>
            </div>
          </div>

<script>
    $("#source").change(function(){
      var id = $(this).val();
      console.log("klik source");
      console.log(id);
      $.ajax({
        type:'POST',
        data:{key:id},
        url: '<?php echo site_url()."table/get_scheme";?>',
        success: function(data){
          $("#scheme").html(data);
        },error: function(){
          alert('error, Please check your internet connection!');
        }
      });

    });
</script>