Flat spins are the worst and most dangerous type of spin. In a flat spin, the aircraft has no forward airspeed. It spins around its vertical axis as it sinks straight towards the ground.

Due to the high angle of attack of a flat spin, it is nearly impossible to recover, as the horizontal stabilizer is most likely in stall, too. The only possibility then is to move the CG forward. In small aircraft what you can do is to lean forward, apply the usual spin recovery as much as possible, and hope for the best.

The type of spin a plane enters depends mainly on the CG Position (and thrust setting):

• CG towards the forward limit makes an aircraft more stable, and control forces will be higher which makes it less likely that large, abrupt control movements will be made. When trimmed, the aeroplane will tend to return to level flight if the controls are released, but the stall speed will be higher.
• CG towards the aft limit decreases longitudinal static stability and reduces pitch control forces, which tends to make the aeroplane easier to stall. Once a spin is entered, the further aft the CG, the flatter the spin attitude.

All in all, do not spin with an aircraft that is not designed for that or with too aft CG position.

Note: Watch the following video to understand the attitude of the aircraft during a flat spin
https://www.youtube.com/watch?v=22eADnRqMwo&t=16s

As for the options:

• “The position of the flaps.” → INCORRECT. This is likely to affect the initial stall characteristics, but not the spin attitude.
• “An imbalance inboard fuel tank quantity.” → INCORRECT. This will affect the direction and initiation of a spin, perhaps, but not the spin attitude.
• “The incorrect use of the ailerons during the recovery.” → INCORRECT. Again, this is likely to be the initial cause of a spin, after recovering incorrectly from a stall, but not the cause of the high nose attitude during the spin.
• “An aft centre of gravity.” → CORRECT.  As mentioned above, a CG at or near the aft limit will lead to a high nose attitude during the spin, which is much more likely to cause the aircraft to flat spin.