答案：C

题目分析：句意：废弃的水在排放时，应该被净化到符合标准。When引导时间状语从句，完整的句子应该为“When the waste water is released, it should be clean enough to meet standards.”当从句主语和主句的主语一致，而且从句谓语动词为be动词时，从句中的主语和谓语动词可以省略，故原句可以省略成“When released, the waste water should be clean enough to meet standards.”故选C。