首先明确一点:remain是个不及物动词.只能说:sb/sth remain.
1.留下,剩下.After the big fire,nothing remained.
2.停留,留下.After the guests left.the remained to clear the room.
这个词为不及物的,也就是说它没有被动语态.
看你的题:剩下的问题.我们把它换成一个定语从句来看;the question that remains.不能说成:the question that is remained.
接下来把定语从句换成非谓语动词,原则你知道的.去连词,去主语,谓语是主动式,换成了remaining.所以说,第一空只能用remaining.
PS:这题要增大难度的话,它应该还有个选项是:left.这词是个及物动词,换成定语从句就是:the question that is left.变成非谓语动词就是过分:left.
即:the remaining/ left question