选2.sitting;reading .
理解本句的关键在于“the girl they had left behind”,其中的“ they had left behind”做定语,修饰先行词"the girl",前面省略了关系代词"who",所以我们先把句子写完整就可看清楚了：the girl who they had left behind(他们遗留下的那个女孩).因此整句可以这样理“他发现他们遗留下的那个女孩正坐在一个角落里读书.”
另外,要掌握“find sb.doing sth.(发现某人正在做某事)”这个句型.