our teachers always let us keep it( )mind ( )we should stick together in face of difficulty
a in which
b in that

如果单纯看，keep it in mind that
后面that指代的句子是定语还是宾语补足语呢？

楼上的回答不正确啊。

这个that引导的从句既不是定于从句，也不是宾语补足语从句，而是一个宾语从句，作动词keep的宾语，而it则为形式宾语，从句才是真正的宾语。这里用形式宾语的原因是为了避免句子的“头重脚轻”。

再举些相似的习题：

• I like _______ in the autumn when the weather is clear and bright.
(A) this (B) that (C) it (D) one
• I hate ________ when people speak with their mouths full.
(A) it (B) that (C) which (D) what
• I hate ________ when I have to speak the local dialect while shopping.
(A) it (B) that (C) which (D) what
• Why don’t you bring ________ to his attention that you are too busy to do it?
(A) me (B) it (C) that (D) yourself
• John had often heard ________ said that Marley has no money.
(A) ones (B) another (C) it (D) that
• You may depend on ________ that your parents will help you whatever you need.
(A) them (B) yourself (C) it (D) me
• He tried to ________ that what he had said was true.
(A) make known (B) make us known (C) make it known (D) make us know (E) make know (F) make it know

by the way, 楼上说“宾语不足语不用从句来表示”也是不正确的，这里有一个例子：
I left the pen where you asked me to.
leave + 宾语 + 宾补的结构中，宾补用了从句。