选择 B if not carefully dealt with
解析：
1、if not carefully dealt with 其实是 if (it is) not carefully dealt with 的省略变形,去掉括号中与主句主语一致的 it 和助动词 is,直接用过去分词短语 if not carefully dealt with 作条件状语.
2、副词性的分词短语可以作时间、原因、条件状语,因此同类的时间状语从句只要主从句的主语一致,都可以用省略主语与助动词（如果有）的“连词 + 分词短语”表示.如：
* I'll call you when I get there.可以改写为 I'll call you when getting there.我一到那里就给你打电话.——去掉主语 I,把原谓语动词 get 改为现在分词 getting（原句没有助动词）
* John didn't attend the meeting because he was ill.可以改写为 John didn't attend the meeting because being ill.约翰没有出席会议,因为他病了.——去掉一致的主语 he,把谓语 was 动词改为分词 being.
* She will play the piano only if she is paid.改写为 She will play the piano only if paid.只有付给她报酬,她才愿意演奏钢琴.
3、如果其中包括否定词 not,hardly 等,其位置要出现在分词前或者是修饰该分词的副词前,如本句 if not carefully dealt with,not 放在修饰过去分词 dealt 的副词 careflly 前.