I am going to visit the farm.My uncle works there.合并成含有定语从句的复合句am going to visit the farm on which my uncle works请问为什么这样改啊,可以改为I am going to visit the farm which / that my uncle works there

My uncle works there中的there其实就是on the farm。
因此，将两句合并为定语从句，前一句的the farm为先行词，后一句中的the farm用which替换，成为：
I am going to visit the farm which my uncle works on.
按照习惯，将介词与其宾语放一起，改为：
I am going to visit the farm on which my uncle works.

你的修改时可以的

当然不可以!
题目的意思是要你运用定语从句讲两句话合并为一句话,需要使用关系代词或关系副词（具体是看引导词在从句里是什么成分）
I am going to visit the farm.我将要去参观这个农场.
My uncle works there.我叔叔在那里工作.
显然第二句的works的地点就是我要去的地方,所以用第二句来做第一句的定语从句.
先行词确定为the farm,而在从句里是需要有一个引导词来做地点状语的,也就是要选一个关系副词where
I am going to visit the farm where my uncle works.
因为某一个特定的关系副词往往可以用相应的介词+which来替换,这里的
where=on which（介词之所以选on,是因为“在农场”要说“on the farm”,而不是in）
所以,句子可以是：
I am going to visit the farm on which my uncle works.
或：I am going to visit the farm which my uncle works on.
或：I am going to visit the farm where my uncle works.

这是个定语从句，你应该看看定语从句的用法
这是表示地点的定语从句
where =on which
哪里的意思
work on the farm, 这里把on提前了，相当于分割了。