Universal Containers requested a custom field on the account to be created to display the number of open cases related to the this requirement?

Universal Containers requested a custom field on the account to be created to display the number of open cases related to the this requirement?
A . Use a flow to populate the custom field value.
B . Use the process builder to populate the custom field value.
C . Use scheduled apex to populate the custom field value.
D . Create a roll-up summary field.

Answer: D