Select box Ajax
============
 $("#edit-model").change('change',function(){  
    var option=$('#edit-model').val();
	 if(option == "Add")
	 {
		 $("#searchShowHide").show();
	 }
	 else
	 {
		 $("#searchShowHide").hide();
	  $.ajax ({
	         type: 'POST',
              url: Drupal.settings.basePath+"?q=dropdown",
             data: {option:option},
             success: function(response) 
			{
                
				 $('#edit-varint').html(response);
				
            }
        });
	 }
	});



menu link
==========
 $items['dropdown'] = array(
    'title' => 'dropdown',
    'page callback' => 'getvariants',
	'page arguments' => array(),
	'access arguments'=>array('access content'),
    'menu_name' => 'main-menu',  // This is the changed line.
    'type' => MENU_CALLBACK,
  );




Function
========
function getvariants()
 {
	$name=$_POST['option'];
	$result=db_query("select  a.field_variant_name_value as name,a.entity_id as id from field_data_field_variant_name a,field_data_field_model_variant b,field_data_field_newmodel c where b.entity_id =a.entity_id  and c.entity_id=b.field_model_variant_nid and c.entity_id='$name'"); 
	print "<option value='0'>Select a Variant</option>";
	if($result->rowCount()>0)
	foreach($result as $row)
	{
		print "<option value='".$row->id."'>".$row->name."</option>";
	}
	print "<option value='Add'>Add a Variant</option>";
	 
 }
 
 
 
 legends
 ========
 edit-model---first selectbox id
 edit-varint--second selectbox id