Sometimes he drops in on his friends after supper.这一题还有个选项so

不可以,因为seldom意思是“很少”,它相当于一个否定词,否定词放在句首,主语和谓语必须倒装,成为：Seldom does he drop in on his friends after supper. （注意：助动词does提到主语之前了）
如果不在句首,倒是都可以：
He sometimes drops in on his friends after supper.
He seldom drops in on his friends after supper.
但单选题的话,老师不会这么出题,因为这样答案就不是唯一的了.