James Baldwin`s plays and short sstories, (which are) to (some degree) antobiographical,established (them) as a (leading) figure in the United States civil rights movement.
不从句意上去理解也可以判断一下,主句主语是作品,中间插入语的主语也是作品,插入语的谓语是复数。如果主句用themselves作宾语就表明主句宾语等于主句主语,而宾语补语的的数是根据宾语来定的,如果这里宾语等于主语,那么宾补的数就应该与主语的一致,但这道题宾补里面用的是a leading figure,显然与主语的数是矛盾的。