A. Scarification: Hard, impermeable seed coat is weakened or ruptured by filing, chipping, hot water
B. Vernalisation- The cooling of seed during germination in order to accelerate flowering when it is planted.
C Impaction- It means to strike something with force. This method implies to the vigorous shaking of the seeds
D. Stratification: Seeds are moistened and exposed to oxygen for variable period at very low (chilling) or high temperature.
So, the correct answer is 'Stratification'.