From CAP 696: Centre of Gravity (CG) is that point through which the force of gravity is said to act on a mass.

Looking at the answer options:

The CG is a theoretical point stated in the Limitations section of the Aircraft Flight Manual --> INCORRECT. The CG will have a safe range specified in the AFM, not just a 'point'.

The CG is a fixed point through which the three axes of the aircraft act --> INCORRECT. The CG is NOT a fixed point, it will move position depending on the load.

The CG is a theoretical point where all the mass is said to be concentrated --> CORRECT. More info below.

The CG is a fixed point defined by the Operator and stated in the Operations Manual --> INCORRECT. The CG is NOT a fixed point, it will move position depending on the load.

The Centre of Gravity (CG) is the point along the longitudinal axis about which an aircraft would balance if it was possible to suspend it at that point. It is the mass centre of the aircraft or the theoretical point at which the entire mass of the aircraft is assumed to be concentrated and acts vertically downward, parallel to the gravity vector in a level flight or in a steady climb or descent.

The CG is not necessarily a fixed point, its location depends on the distribution of weight in the aircraft, but must remain within certain forward and aft limits, for reasons of stability, control and structure, considering both the flight and ground requirements.

Also, the Limitations section contains only those limitations required by regulation or that are necessary for the safe operation of the aircraft, power plant, systems, and equipment. It includes operating limitations (such as CG and Load limits), instrument markings, colour-coding and basic placards.