A positive edge$-$triggered D flip$-$flop will store a $O$ when

a$.$ The D input is LOW and the clock is HIGH

b$.$ The $D$ input is LOW and the clock transitions from HIGH to LOW

c$.$ The D input is HIGH and the clock transitions from HIGH to LOW

d$.$ The D input is LOW and the clock transitions from LOW to HIGH

e$.$ The D input is HIGH and the clock is LOW