A D flip-flop is used in a 4-bit serial adder, why

a-d-flip-flop-is-used-in-a-4-bit-serial-adder

A D flip-flop is used in a 4-bit serial adder, why?
A. It is used to invert the input of the full adder
B. It is used to store the output of the full adder
C. It is used to store the carry output of the full adder
D. It is used to store the sum output of the full adder

The correct choice is C. It is used to store the carry output of the full adder