Why A shift register is a chain of flip-flops wired so the output of each stage feeds the input of the next. All stages share a common clock, so on every clock pulse the stored bits move one position down the line. That makes shift registers useful for serial-to-parallel conversion, delays, and moving data one bit at a time.
Watch out The op-amp answer confuses digital logic with analog circuitry, and the mixer choices describe circuits that combine signals rather than store and move bits; no mixing takes place in a shift register.
Shift register = flip-flops in a line, clock ticks, bits march one step.