the police finally found the robber lying near the river,___

答案是A.dead.
这是形容词作伴随状语的情况,在英语中很普遍.这里再给出几个形容词作伴随状语的例子：
Those soldiers went back to their country, tired but safe.
注意tired和safe是形容词.
又如：
The man seated himself there, silent.
这种形容词作伴随状语实际上是对主语伴随情况的补充说明,相当于and + 主语 + be .例如题句可理解为：
The police finally found the robber lying near the river, and he was dead.
我上面的例句也可以理解成：
Those soldiers went back to their country, and they were tired but safe.
不知我的回答是否能让你满意.
祝你快乐!