d) Yixi should use Spring Y. Based on the results in the table, Spring Y is stiffer than Spring X as it compressed less when both springs were subjected to the same load. Therefore, more force is required to compress Spring Y to the required length under the metal disc as compared to Spring X. There will then be more elastic spring force acting on the metal disc to push the next candy upwards when a candy is removed.

@Adwin Thank you!