此句是否省略了that
热心网友
Perhaps the most difficult challenge is that how to survive without friends. 是,宾语从句!
热心网友
是表语从句，Perhaps the most difficult challenge是主句，is how to survive without friends是系动词加上表语从句，that引导表语从句可以省。
热心网友
这是同位语从句。
热心网友
Perhaps the most difficult challenge is how to survive without friends.这里的how to survive without friends引导表语从句.不需要that
热心网友
没有省略how to survive without friends在be动词后边，是表语从句。已经有how引导，不必再用that引导
热心网友
不是.that引导的是陈述句.