参考答案

正确答案：an advantage
an advantage 解析：根据关键词the Tale of Two Pebbles，reverse定位到第五小标题下最后一段末句：Since the moneylender couldn't admit what he had done，the girl effectively turned what looked like an impossible situation into an advantage．可知，小姑娘最后成功扭转了局势，转不利条件为有利条件。题干中reversed the unfavorable condition与原文turned what looked like an impossible situation为同义转述，故此处空缺an advantage。