a. A printout of all sales taken from the register at the end of the day Amazon providing you a list of products similar to others you've viewed
Explanation:
A recommendation engine is something which according to its user's preference provide them a list of similar objects. From various platforms according to its user's preference.
Similarly, Amazon shows you a variety of previous products which you have purchased. So, in this case option a is most appropriate. Here Amazon will first observe different sales trend from the register then as a recommendation engine it will provide you with a list of products similar to others you have viewed.
