The optimal way to implement this feature is to build a flow with Flow Builder. Flow Builder is a tool that lets you automate business processes by creating flows that execute logic, interact with Salesforce, and call Apex classes. You can use Flow Builder to create a record-triggered flow that runs when an Account record is updated. In the flow, you can use a Get Records element to query the custom object based on the Postal Code field of the Account record. Then, you can use an Assignment element to assign the value of the Timezone field from the custom object record to the Timezone field of the Account record. Finally, you can use an Update Records element to save the changes to the Account record.
Option B is incorrect because an account assignment rule is used to automatically assign account owners based on criteria. It does not update other fields on the account record.
Option C is incorrect because a formula field is a read-only field that derives its value from a formula expression. It cannot be updated by the user or by a trigger.
Option D is incorrect because an account approval process is used to require approval from one or more users before an account record can be saved. It does not update other fields on the account record.
References: Flow Builder (Trailhead)), Record-Triggered Flows (Trailhead)), Account Assignment Rules (Salesforce Help)), Formula Fields (Salesforce Help)), Approval Processes (Salesforce Help))