Option B. Roll-up summary field
is the correct answer.
Roll-up summary field is created for the purpose that we could be able to calculate the figures and values from the related records or from related lists. Another purpose of creating it is to display a record in Master record that is present already in Detailed Record.
A roll-up summary field has summary of all the records that make it possible to review the values and perform calculations using these values. This field allows to perform many calculations depending upon the need.