The correct answer is D. The class quickly learnt the major points of the lesson but also forgot them quickly.
Option A is incorrect because it does not include the fact that the major points of the lesson were also forgotten.
Option B is incorrect because it does not include the fact that the major points of the lesson were quickly forgotten.
Option C is incorrect because it does not include the fact that the major points of the lesson were both learnt and forgotten.
The sentence “The major points of the lesson were quickly learnt by the class,but they were also quickly forgotten by them” can be paraphrased as “The class quickly learnt the major points of the lesson, but they also quickly forgot them.” This means that the class learnt the major points of the lesson quickly, but they also forgot them quickly.