Answer: (D)
explanation:- let z=Qn
now we will put X=k and Y=j
after solving first AND gate we got $\bar{k}Qn$
after solving second AND gate we got $J\bar{Qn}$
after solving OR gate we got $J\bar{Qn}\dotplus \bar{K}Qn$
$\because$ we knows that in D flip flop next output(Qn+1)=D
after solving D flip flop we got Qn+1=$J\bar{Qn}\dotplus \bar{K}Qn$
this is the characteristic equation of J-K flip flop…..hence option (D) is correct.