In a JK flip flop the output toggles when both J and K inputs are $1.$ So, we must ensure that with each clock the output from the previous stage reaches the current stage.
Setup time is defined as the minimum amount of time before the clock's active edge that the data must be stable for it to be latched correctly
It is given that set up time is negligible - means as soon as data is stable, next clock can be given.
Time to get output from $FF$ once input (and clock) is given = $10ns.$ (Propagation Delay)
Time for inputs to reach $FF_1 = 0.$ (Zero AND gate)
Time for inputs to reach $FF_2 = 10.$ (One AND gate)
Time for inputs to reach $FF_3 = 20.$ (Two AND gates)
So, minimum time period needed for clock is $10 + \max(0,10,20) = 10 + 20 = 30ns$ which would mean a maximum clock frequency of $1/30 GHz = 33.33 MHz$