选择C
They lost their way in the mountain,and 【what】made matter worse was that night was coming.
他们在山里迷路了,跟糟糕的是夜晚要降临了.
【主语从句what made matter worse 】【系词was】【表语从句 that night was coming.】
what~是主语从句,在从句部分中作主语.