Two considerations that the administrator should remember about roll-up summary fields are:
Roll-up types include COUNT, SUM, and AVG. A roll-up type is a function that determines how the data from the child records is aggregated and displayed on the parent record. The available roll-up types are COUNT, which counts the number of child records; SUM, which adds up the values of a numeric field on the child records; AVG, which calculates the average value of a numeric field on the child records; MIN, which displays the lowest value of a field on the child records; and MAX, which displays the highest value of a field on the child records.
Roll-up summary fields are created on the master side of a master-detail relationship. A master-detail relationship is a type of relationship that links two objects together such that the master object controls certain behaviors of the detail object, such as security and deletion. A roll-up summary field is a type of field that displays a value that is calculated from child records related to a parent record. Roll-up summary fields can only be created on the master object of a master-detail relationship or on a lookup relationship if it is set as required.
The other two options are incorrect because:
Roll-up summary fields do not prevent the conversion of a master-detail relationship to a lookup. They only prevent it if they reference a formula field that includes another relationship.
Rollup fields are not calculated prior to save. They are calculated after save and may take some time to update.