Many people have realized that we won't have any more clean

答：时间和条件状语从句中用现在时态表示将来,人称“主将从现”.
例：I will tell him about it if(/when) I see him tomorrow.
本题Many people have realized that 是主句,为现在完成时,不影响that-宾语从句的谓语动词.可忽略不看.只看下一句就行：
We won't have any more clean water to drink unless something _____ to fight against pollution.
unless=if...not...是引导条件状语从句的.
B是将来时,A是过去时,C是非谓语,可排除.
D.是一般现在时的被动语态.
unless sth.is done to fight against pollotion= If sth.is not done to ...
=if we don't do sth.to fight.
译：许多人已经意识到,如果不采取措施与污染作斗争,我们将不再有干净的饮用水可喝.