This is one of the happiness days that ____been spent in my life.___中填 have 还是 has 为什么?

have 因为 that 作为关系代词 代替 days 根据主谓一致的原则 谓语动词用复数形式
倘若 one 的前面 加上定冠词 the 答案则是 has 因为 此时 that 代替的是 the one 谓语用has

have

填 have
that have been spent in my life是定语从句 作定语修饰 days ,也就是说,先行词是days,复数形式.that指代的是days,所以要用have
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have
因为这是个that引导的定语从句，而that指代的是days，所以用have