Question: Which one is the correct way of asking this question? When? 

a) Why you changed your job?

b) Why did you change your Job?

c) Let me know why you changed your job?

d) Let me know why did you change your job?

Answer: 
Not in Standard English. Main clause interrogatives like this require subject-auxiliary inversion: "Where did you bring all these vegetables from?" Note that the plain verb-form "bring" is required after the auxiliary verb "did".

(answer transcribed from comment)
The format of the question is not quite correct. You need to have: 

* auxiliary verb + subject + verb

So, we would say: 

> 
> Where **did** (*aux verb*) **you** (*subject*) **bring** (*verb*) all these vegetables from?
> 
> 
>