Natural methods (such as the rhythm method or periodic abstinence) work on the principle of avoiding the meeting of ovum and sperm. The text explicitly states that for these methods, "chances of failure, though, of this method are also high" . In contrast, surgical methods like vasectomy are described as "highly effective" , pills are described as "very effective" , and IUDs are described as "effective and popular" . Therefore, the rhythm method is the least effective among the options.