Explanation:

- Correct Answer: Option 3: Flip-flop

- A Flip-flop is a basic memory element used in sequential logic circuits to store a single bit of information. It's used in registers for state retention.

- A Register consists of several flip-flops and is used to store multiple bits, not just one. It's a group of flip-flops.

- A Transistor is a basic building block of electronic circuits. It controls voltage but does not store information by itself.

- A Capacitor stores electric charge but is not used for long-term information storage as a flip-flop is.