the answer C is correct, you can't schedule a roll-up summary

D. Use a Roll-Up summary.

Explanation:
To count the number of open cases associated with each account and update the account with this value on a schedule, the administrator should use a Roll-Up Summary field.

A Roll-Up Summary field allows administrators to calculate a value from related records and store that value on the parent record. In this case, the administrator can create a Roll-Up Summary field on the Account object that counts the number of open cases associated with each account. This field can be set to update on a schedule, such as every Friday evening.

Using a record trigger flow or scheduled flow may be possible, but it would require additional development work and may not be necessary for this scenario.

Using a scheduled process builder can also be a possible solution, but Roll-Up summary would be more efficient as it is a native feature in Salesforce and would require less configuration and maintenance effort.

Overall, using a Roll-Up Summary field is the simplest and most efficient way to count the number of open cases associated with each account and update the account with this value on a schedule....