The correct answer is B. Flow.

The requirement is to show the number of open Cases for both the Account and the Parent Account. This is not a simple native roll-up summary scenario because standard roll-up summary fields are mainly for master-detail relationships, while lookup relationships do not support standard roll-up summary fields.

Salesforce specifically documents Flow examples where a flow looks up the parent account, checks how many open cases it has, and evaluates that count. Salesforce also has guidance for creating roll-up-style summaries using Flow, including a use case to count Cases related to an Account.

Why others are incorrect:

A. Approval Process is for approvals, not counting open Cases.

C. Custom object is unnecessary.

D. Roll-up summary is not the best fit for counting Cases across Account and Parent Account relationships.

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