I cant select options from autocomplete textbox

Question

$('#txtmn').autocomplete({

source: '@Url.Action("GetMaterialNums")',

minLength: 1,

selectFirst:true

});

 

public JsonResult GetMaterialNums(string term)

{

List<string> Mnums;

Mnums = db.Server_SparesAdd.Where(x => x.MaterialNo.StartsWith(term)).Select(x => x.MaterialNo).ToList();

return Json(Mnums, JsonRequestBehavior.AllowGet);

}

   Please give me any suggestions to solve this issue