In the following question a sentence is divided into four parts A, B, C and D. Rearrange it in the proper sequence in order to make a meaningful sentence:
A. or unfair treatment
B. affirmative action is a policy
C. that tries to make up for past discrimination
D. against certain groups of people

The starting part should be B, as the idea of affirmative policy is introduced.
The following part should be C, since the pronoun "that" refers to "affirmative policy".
Part A is having a connector "or" which connects "discrimination" and "unfair treatment".
So, part A should follow C.The concluding part is D, which tells us against whom this discrimination or unfair treatment occurs.

So, the correct order is B-C-A-D.