Which approach should the developer use to accomplish this declaratively?

A developer working on a time management application wants to make total hours for each timecard available to application users. A timecard entry has a Master-Detail relationship to a timecard.

Which approach should the developer use to accomplish this declaratively?
A . A Visualforce page that calculates the total number of hours for a timecard and displays it on the page
B . A Roll-Up Summary field on the TimecardObject that calculates the total hours from timecard entries for that timecard
C . A Process Builder process that updates a field on the timecard when a timecard entry is created
D . An Apex trigger that uses an Aggregate Query to calculate the hours for agiven timecard and stores it in a custom field

Answer: B