--What do you think of teaching,Bob?
--I find it fun an challenging. It is a job______ you are doing something serious but interesting.答案为where，为什么？

这是一道定语从句的题。you are doing something serious but interesting修饰job。意为“在这样的工作中你能做一些严肃而又有趣的事”。

确定为定语从句后，可把先行词（被修饰的词，此处为job）试着放到后面的从句中。如果不需要添加介词就可以完整的表达一句话，连词就选用关系代词：that， which， who，whom；如果需要补充介词才能成句，则用介词+关系代词，而且，此时若先行词为表示时间、地点、原因的词，也可以相应的用关系副词when，where，why。
如：This is the factory ______ I visited last year.
(which)
(I visited the factory last year.)
This is the factory ______ I worked last year.
(in which或用where）
(I worked in the facotry last year.)

It is a job…的这道题也一样。