peter told them that ----ask them to hand in the papers, the

B容易排除,整个句子的时间都是过去式,选B的话动词就是ask,原形,行不通.
“如果……”表示对将来虚拟的有三个常见句式：
If+主语 +过去式
If+主语 +should do
If+主语 +were to do
A 选项正确的表达 if someone asked them to hand in the papers
D选项应该是if someone were to ask them to hand in...虚拟语气中用were而不用was
C选项是正确的,这是省略句倒装的情况,本来句子是if someone should ask them to...,省略了if之后,should应该提到句首,就变成了should someone ask them to...
建议你可以温习一下倒装句的知识.