Students’ complaints _____ too much study burden were soon taken into consideration by the school.

choose：A
because complaint is a noun,when it before of,for,in or with,we choose of after it,that means the following content of "of" is the content of complaint.On the contrary,we choose for,to say the content before "for" means the content of the complaint.
因为complaint是名词,当它在of、for、in或者with前面时,我们选择of,用以表示所抱怨的是什么内容.反过来,当它在后面时,表示for之前的内容即是所抱怨的内容.
而当它与in啊,with啊在一起时,complaint更多的表示投诉啊,申诉之类的意思.
显然在这里选择A最妥当,C和D不合语境.