validate a BD data and show results in an option

The detail I have is that I have to validate a text (RFC) of an input, and that when clicking on a submit check, I return the clients with the same RFC in an option .... I already look and I do not understand how to do it ...

this part is the search with the SQL query

<label> 
      <select id="cbx_nombre" name="cbx_nombre">
                    <?php

                    include "conexion.php";
                    $consult = "SELECT *from usuarios3 where RFC = 'RFC123' "; /*AQUI VA EL RFC*/
                    $ejecutar=mysqli_query($conexion, $consult) or die (mysqli_error($conexion));
                    ?>
                    <option>Selecciona::</option>
                    <?php
                    foreach ($ejecutar as $opciones):?>

                        <option value="<?php echo $opciones['id_cliente'] ?>">
                            <?php echo $opciones['id_cliente'] ?>
                </option>

                    <?php endforeach ?>


            </select>
        </label>

I know that with jquery, but I do not understand 100 well how could I do that ...

    <script type="text/javascript">
        $(document).ready(function(){

      var consulta;

      //hacemos focus
      $("#usuario").focus();

      //comprobamos si se pulsa una tecla
      $("#usuario").click(function(e){
             //obtenemos el texto introducido en el campo
             consulta = $("#usuario").val();

             //hace la búsqueda
             $("#resultado").delay(1000).queue(function(n) {      

                  $("#resultado").html('<img src="ajax-loader.gif" />');

                        $.ajax({
                              type: "POST",
                              url: "comprobar.php",
                              data: "b="+consulta,
                              dataType: "html",
                              error: function(){
                                    alert("error petición ajax");
                              },
                              success: function(data){                                                      
                                    $("#resultado").html(data);
                                    n();
                              }
                  });

             });

      });

});
    </script>