How can this requirement be met?

The Marketing Manager has requested that a field be added to each account that displays the number of contacts associated with the account. The manager wants to use this field as part of an email marketing segmentation strategy.

How can this requirement be met?
A . Create a roll-up summary field that counts the number of contacts and displays the count on the account.
B . Create a custom formula field on the account using the count() function to count the number of related contacts.
C . Create a custom field on the account. Use an Apex trigger to update the field when contacts are added or deleted.
D . Create a custom field on the account. Use a workflow rule to update the field when contacts are added or deleted.

Answer: C