Teaching is not a(n) ____ which pays very well.
A. post
B. occupation

Note: What’s wrong with A if any, thanks!
By the way, I think "which" needs to be replaced with "that," what do you think?

Question

Teaching is not a(n) ____ which pays very well.
A.	post 
B.	occupation

Note: What’s wrong with A if any, thanks!
By the way, I think "which" needs to be replaced with "that," what do you think?

Answer

A post is something you are appointed to.  An occupation is more of something you choose.  Generally, a person chooses to be a teacher, rather than being assigned to be one.

As for replacing "which" with "that", I would agree with you.  If you cannot remove the part after "which" without destroying the meaning of the sentence, you should use "that" instead.