I saw him ----under a tree,thinking.A.sat B.seated C.seating

可以从这个角度看,see 作为感官动词,后面的搭配1,动词的ing形式,表看见某人正在干什么；2,动词的ed形式（过去式）表做过某事.
所以这种一般做题逻辑是没有问题的.
但是,此题考察的重点是单词Seat,seat作为使动动词,用法有二,1．be seated.eg:He was seated there.;2．seat oneself.eg:He seat himself there.
两个语法点叠加在一起,选B.如果出现选C的状况,除非为I saw him seating HIMSELF under a tree,thinking.
此处,需要注意,seat 换为sit时,I saw him sitting under a tree,thinking.完全正确
希望学习时多加注意