C.
A 的话因为是"how"的问题,do要加宾语,"how we should do it" 就对了.
B 是纯问句形式,要倒过来做从句 "what we should do"
C 是 "how" 的问题,do 有宾语,用 "to" 代替 "we should",正确. 也就是说这道填空可以三种说法,但是A,B都有语法错误,3种正确说法是:
No one tells us how we should do it,...
No one tells us what we should do,...
No one tells us how to do it,...