--Thank you for a ____ interesting evening I have never spent.

--My pleasure.

C. most  D. more

这个题为命题错误。无论填most还是more，句子都是错误的。

这里想表达的意思是；感谢你让我度过了一个最有趣的夜晚。这是一个最高级概念，我们可以用最高级来翻译，但也可以用比较级来表示最高级意义。例如：

Thank you for the most interesting evening I have ever spent.

Thank you very much. This is the most interesting evening I have ever spent.

Thank you very much. I have never spent a more interesting evening (than this one).

Thank you very much. This evening is more interesting than any other evenings I have ever spent.

你写的两个句子结构过于复杂，口语中是没有人那样说话的。

than which I have never spent a more interesting one. than必须置于which之前。这种结构偶见于书面语。