答案：C

考查含有if条件状语从句的虚拟语气。根据wouldn’t have told，可知指的是过去情况，所以if从句要用过去完成时态，即if I had known he was seriously ill,助动词had可以提到句首，省略if ，构成部分倒装，即had I known he was seriously ill。所以选C。