选C吧.
By no means / in no case / at no time / in no way等表示“绝不”的短语位于句首时要部分倒装,is提前.
it能做形式主语,代“that all English people know their own language well”,that则不能.
原句：That all English people know their own language well is by no means true.
句意：所有英国人熟知他们的语言是绝不正确(可能)的.