There are about 1,000 students in Grade One ,------took these exams at the end of last term.

D,all of whom 是对的.
句子有逗号,逗号后面又没有连词,说明后面的部分不是一个独立的分句,而只是修饰成分-----这里是个定语从句.
下列几个句子均为正确表达,意思一样.请注意句子中间是句号还是逗号,以及它们后面的首字母的大小写：
There are about 1,000 students in Grade One ,all of whom took these exams at the end of last term.
There are about 1,000 students in Grade One.All of them took these exams at the end of last term.
There are about 1,000 students in Grade One ,and all of them took these exams at the end of last term.