这里选A,用his和后面的not being allowed作为一个动名词组.等于What worried the child most was that he was not allowed to visit his mother in hospital.
B是错误的,因为这里没有完成时的意思,所以不能用having.有一种是 主谓一致的情况下可以省略his 的这种情况是什么？如果把这个句子再改成主谓一致，就是： The child worried most was his not being allowed.....