Starting with the CAP 696 definition:

• 'The Centre of Gravity (CG) is that point through which the force of gravity is said to act on a mass.'

The force of gravity acting on a mass gives us weight. So to simply the CAP definition:

• The CG is the point through which the weight acts.

An aircraft is constructed of many parts, all of which contribute to the weight. The CG is the overall point at which they act along the longitudinal axis.

Looking at the answers:

'The CG is the point at which the sum of the balance arms is nil' --> INCORRECT. Balance arms are just the distance from the datum, not the effect.

'The CG is the point at which the total weight is assumed to be concentrated' --> CORRECT. As described above.

'The CG is the point at which the sum of the values of the moments is NOT zero' --> INCORRECT. At the CG the sum of the weight moments is zero.

'The CG is the point at which the total weight is applied and acts in the longitudinal direction' --> INCORRECT. The weight acts vertically downwards with gravity.