Refer to figure.
Let’s start off by treating each wing individually. During or close to the stall, each wing will be at a very high angle of angle of attack. Any further decrease in airspeed and therefore angle of attack will cause an even more aggressive drop in coefficient of lift.

Now let’s consider a large rudder pedal deflection during the above mentioned scenario. A large yaw will; for the rearwards going wing; cause a decrease in the longitudinal airflow component, without reducing the vertical component (see right side of figure). This will cause the angle of attack of the rearwards going wing to increase. The increase in angle of attack will cause that rearwards wing to have an even lower coefficient of the lift than normal and cause a decrease in lift. This causes said wing to drop.

At same time the front moving wing will experience an increase in longitudinal airflow without any change in the vertical component. This will cause a decrease in angle of attack. Considering that the aircraft is already in a stalling situation which means the angle of attack is higher than the critical angle, this decrease in angle of attack will increase the coefficient of lift and therefore the lift produced by the front going wing.

The extra wing produced by the front moving wing and the reduction in lift produced by the rearwards moving wing will create a roll force that will spin the aircraft.

Therefore, a yaw during slow flight and therefore also stall will cause a spin.

The other answers are incorrect because:

• A forward centre of gravity will decrease the fuel efficiency, and affects the stability but will not cause a spin.
• Maximum load factor exceedance is likely to cause a stall due to higher stall speeds than expected, however a stall does not turn into a spin without asymmetric flight.
• Abrupt use of the rudder during cruise; this is very unlikely to cause a spin because the angle of attack at high speeds during cruise is so far away from the critical angle of attack that a spin would be very unlikely.