You are given a free running clock with a duty cycle of $$50$$% and a digital waveform $$f$$ which changes only at the negative edge of the clock. Which one of the following circuits (using clocked $$D$$ flip-flops) will delay the phase of $$f$$ by $${180^0}?$$

$$SR.$$ latch made by cross coupling two $$NAND$$ gates if $$S=R=0,$$ Then it will result in