all finished,we sat down to enjoy ____we thought the most delicious dinner.

all finished,we sat down to enjoy ____we thought the most delicious dinner.

what we thought as比较好的吧。。
that的话就是to enjoy that most delicious dinner
通常来说两个都可以用，但碰到这样的情况试着把两个词代进去看哪个比较make sense..
比如说we like that cake
we like what is in the cake而不能说we like that is in the cake
we wish that her dreams can come true或者we wish what her dreams can come true
但意思就不太相同了。。
啊，不太会解释额。。抱歉，反正就是那个意思啦。
总之这里用that的话不通

what

what哈
that是那种指代很明确的时候用的
what在这能体现一种抽象感

我觉得这个问题应该从语法的角度去入手哦。
that 一般引导的是定语从句，用于修饰前面的名词
what 引导的是宾语从句。
从句里（即 we thought的分句）是缺少宾语的（也就是我们所以为可口的对象）。如果单说这句话应该是：
we thought the dinner the most delicious(dinner).
所以这里实际上是缺少了dinner这个成分，需要用what来引导为宾语从句，从而使分句的成分完整。

wish the answer is helpful, good luck.