选 B have
解释很简单,两个语法, 一个是how 的 感叹句:how close parents are,to their children 是修饰close的, 另外一个是who引导的从句,who从句类似于:this is Tom who meet you last monday.
你所问的句子可看成两段话:
how close parents are to their children.
Parents (who) have a strong influence...
也就是说 下划线后面一节都是修饰parents的who从句.
既然是修饰parents,那么根据语法结构,从句主谓宾结构,就选 have