比较的对象是to prevent crimes against property和to prevent crimes against a person.
本来是这样的To prevent crimes against property is as difficult as to prevent crimes against a person. 改成it作形式主语就是这样的It is as difficult to prevent crimes against property as it is to prevent crimes/those against a person.因为平行,所以前后都要改成it的形式。
如果句子是Crimes against property are as difficult to prevent as crimest/those against a person (are),那么B就对了。