if D=Vo/Vs

then 0.6=Vo/15

Vo=9v?

this seems like a trap

 

Your answer looks fine.

 

answer C 9V is incorrect.... It was a trap I knew it!!!!

 

The correct answer is D because it operates in the discontinous conduction mode (DCM).
Assuming CCM operation, the maximum inductor ripple current at D=0.6 is 2.4Ap-p (in CCM) and the output current is 0.6A (9/15). Thus it is not possible to operate in CCM since the output current is less than 2.4/2. Thus, the output voltage shall be greater than 9V. Answer E is impossible, thus answer D is correct.

If you want to calculate it:

http://en.wikipedia.org/wiki/Buck_converter