The answers to this question are B, C, and E. A roll-up summary field is something that makes use of various records in order to do calculations. The best thing about this is that it can display the values that are needed in a limited amount of time. There is no need to wait a long time anymore or to compute one by one just to get the right answer.
Formula fields cannot be calculated especially if they do not have the proper functions that will make them easy to compute. Formula fields also do not contain cross-objects that will supposedly make them easier to use and compute. Take note that there is a limit of up to 10 roll-up summary fields for every object available.