In which of the following times would it be okay $($i$.$e$.,$ not cause problems$)$ to see a floating value on the D$-$input to a D$-$flip$-$flop?

a$)$ You should never have a floating value on the D$-$input

b$)$ After the active clock pulse, as long as it is not floating when the next active clock pulse occurs

c$)$ Anytime in which the flip$-$flop is enabled for storage

d$)$ At the time of the active clock pulse

e$)$ At the time of the NGT clock edge