1.我估计楼主是想找出定语从句的宾语,而不是主句的宾语.
如果关系代词在定语从句中充当动词的宾语,则关系代词可以直接省略；
如果关系代词在定语从句中充当介词的宾语,而且介词并没有和该关系代词一起提前至定语从句句首时,则关系代词也可以省略.但是如果该介词和关系代词一起提前至定语从句句首时,则关系代词不能省略.
2.答案如下：
1）关系代词which = 先行词the power station,在定语从句we visited中充当visited的宾语,可以省略；
2）关系代词which = 先行词a power station,在定语从句burned coal中充当burned的主语,不能省略；
3）关系代词which = 先行词coal,在定语从句came from the northwest of the country中充当came from the northwest of the country的主语,不能省略；
4）关系代词who = 先行词the people,在定语从句built the Aswan High Dam 中充当built the Aswan High Dam的主语,不能省略；
5）关系代词that = 先行词a town,在定语从句was submerged when they built the dam中充当was submerged的主语,不能省略；
6）关系代词that = 先行词the temple,在定语从句we visited first 中充当visited的宾语,可以省略；
7）关系代词who = 先行词the woman,在定语从句we met at the temple中充当met的宾语,可以省略；
8）关系代词which = 先行词the carvings,在定语从句we saw on the cliff 中充当saw的宾语,可以省略.