如果还原此句你就知道选那个了
his is much the best of all the suggested plans
his is the best by far of all the suggested plans
by far表示“……得多,最……,显然,绝,远远地.大大地.尤其.在很大程度上等”,用来修饰比较级或最高级,强调数量、程度等.
如果选b,此句应该改成
his is by far the best of all.
or
his is the best of all.by far
换句话说b错在by far的位置不对