what if we meet a situation _________none of us are able to deal with?

定语从句，that引导从句 修饰situation，用that的原因是，situation是一种情况，that可以修饰“人或物” who \whom修饰人的，which and what 都不符合句意。
如果我们遇到一种我们所有人都解决不了的情况，怎么办呢？

that

that
后面加从句

that 带从句 none of us are able to dearl with 来解释situation

What if we meet a situation that none of us are able to deal with?
如果我们遇到我们不能应付的情况，会怎么样呢？

是定语从句。