Which one?.

d. Clock

Explanation:

A flip flop is an circuit, an electrical circuit precisely that is used as a means to story data or information. This type of data or information stored in a flip flop is binary data or information. A flip flop has a high stable state and a low stable state.

A flip flop is a circuit that is very essential and must be present when building systems like the computer or communication system.

A flip flop must have a CLOCK signal or indicator because this helps control the triggering(i.e the change from one state to another) that occurs in a flip flop.