The code snippet that should be added to the setup for the unit test to create the controller extension for the test is option A. This code snippet creates a PageReference object that points to the Visualforce page that uses the controller extension. Then, it sets the current page reference to the PageReference object and passes the Account ID as a URL parameter. Finally, it creates an instance of the controller extension by passing the standard controller as an argument. The code snippet in option B will not work because it does not set the current page reference or pass the Account ID as a URL parameter. The code snippet in option C will not work because it does not create a PageReference object or set the current page reference. Reference: [Testing Custom Controllers and Controller Extensions], [PageReference Class], [ApexPages Class]

The isCustom() method is a method that can be used to determine whether an sObject is a custom object or a standard object. The method returns true if the sObject is a custom object, and false if the sObject is a standard object. The method can be invoked on the sObject describe result, which is an object that contains the metadata information about an sObject, such as its name, label, fields, and relationships. By using the isCustom() method on the sObject describe result, the developer can check which objects to include in the response to the Lightning web component, and filter out the standard objects if needed. Reference: [isCustom() Method], [sObject Describe Result]

The seeAllData=true annotation is used to allow a test class to access all data in the org, instead of using test data that is created in the test class or by using test setup methods. This annotation should be used sparingly and only when absolutely necessary, as it can make the test class dependent on the data in the org and cause unexpected failures or unreliable results. The seeAllData=true annotation is required for some sObject types that do not support test data creation, such as Report, Dashboard, Document, and Attachment. Therefore, if the test class needs to access a Report sObject, it must use the seeAllData=true annotation. Reference: [Isolation of Test Data from Organization Data in Unit Tests], [Using the seeAllData Annotation]

The three changes that the developer should implement in the Apex class to ensure the LWC can display data efficiently while preserving security are to annotate the Apex method with @AuraEnabled, implement the with sharing keyword in the class declaration, and annotate the Apex method with @AuraEnabled(Cacheable=True). The @AuraEnabled annotation allows the Apex method to be called from a Lightning web component, and exposes the method to the Lightning Data Service. The with sharing keyword enforces the sharing rules and permissions of the current user when accessing the records, and ensures that the data is secure and compliant. The @AuraEnabled(Cacheable=True) annotation enables the Apex method to be cached by the Lightning Data Service, and improves the performance and efficiency of the data retrieval. Implement the without keyword in the class declaration is not a valid answer, as it ignores the sharing rules and permissions of the current user when accessing the records, and compromises the data security and compliance. Use the WHERE clause within the SOQL query is not a valid answer, as it is not a change in the Apex class, and it does not affect the security or efficiency of the data display. Reference: [Call Apex Methods], [Using the with sharing or without sharing Keywords], [Improve Performance with Storable Actions], [Apex Developer Guide]

Based on the code snippets, the change that should be made to display the contacts' mailing addresses in the Lightning web component is to modify the SOQL query in the getAccountContacts method to include the MailingAddress field. This way, the Apex controller class can return the contacts' mailing addresses along with their names and phone numbers to the Lightning web component. The Lightning web component can then use the @wire annotation to get the data from the Apex method and display it in the lightning-datatable component. Adding a new method in the Apex controller class to retrieve the mailing addresses separately and modifying the Lightning web component to invoke this method would be inefficient and unnecessary, as the mailing addresses can be retrieved in the same query as the other fields. Extending the lightning-datatable component in the Lightning web component to include a column for the MailingAddress field would not work, as the data for that column would not be available from the Apex method. Modifying the SOQL query in the getAccountContacts method to include the MailingAddress field and updating the columns attribute in javascript file to add Mailing address fields would work, but it is not the optimal solution, as the columns attribute is already defined in the HTML template file and does not need to be duplicated in the javascript file. Reference: [Call Apex Methods from Lightning Web Components], [lightning-datatable]