Afer graduation I'd like to find a job,--where I can do some

B.one 为正确选项.
这里的one是做前面a job 的同位语,而where I can do something serious but interesting 是定语从句,修饰one的.where是关系副词,=in which,in which I can do something serious but interesting 逻辑上=I can do something serious but interesting in the job.
A.the one 错在,不是特指,不能用the；C.which 和 D.that 也错,因为定语从句不缺成分,不能用它们.