I'll try. It's hard because I can't use a subscrpt. Here we go..
REACTION....
A2 (aq) + 2B- ---> B2 (aq) + 2A- (aq)
C2(aq) + 2B- (aq) ---> no reaction
D2 (aq) + 2A- (aq) ---> 2D- (aq) + A2 (aq)
Anything after a letter is an small number, and the negatives you see behind some of the terms are ion charges.
The question is when these elements are arranged in order from strongest oxidizing agent to the weakest oxidizing agent, the order is...
Using these...A2, B2, C2, D2
The answer is D2, A2, B2, C2.
I understand why C2 is last, because there is no reaction, but I don't know how to figure out the other three!