1,He was often seen -----with the peasants, A, work B, worki

was表示过去发生的动作,see sb。do sth 注意,一般情况下不表示看到某人经常做某事。它表示你看到过某人做某事,然后often表示经常。用see sb doing 是指你看到某人正在做某事或者表示看到某人经常做某事就像be always doing 一样的用法。排除a 。c 和 d就不啰嗦了。
还原成主动语态someone often saw him working with the peasants