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