The young man was often seen -C---- by the lake.

试题就是考see sb do 的结构
1）感官动词see,watch,smell,notice等在主动语态时后接动词do或者doing
see sb do sth-------------被动式为sb be seen to do.
see sb doing--------------被动式为sb be seen doing.
 see sb do sth表示“看见某人做了某事”,强调看到某人做完了某事（强调结果）
 see sb doing sth表示“看见某人正在做某事”,强调某人正在做某事(强调过程)
比较下列两句：
I saw him repair his bike. 我看见他修了他的自行车.（他已修完了自行车）
I saw him repairing his bike. 我看见他正在修他的自行车.（强调看见正在修车）
2） 所以题目The young man was often seen ____ by the lake.
换成主动态： He often saw the young man____by the lake.
显然主动态应该填 draw,句子强调的是看见动作的结果,而不是过程.
被动态就为： be seen to do ,自然选A