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To meet the given requirements, we need a relationship that allows:

Calculating the total amount on an Order.

This can be achieved using roll-up summary fields on the Order object to sum up values from its Line Item child records.

Roll-up summary fields are only available on the master object in a master-detail relationship.

The Apex test fails in the Test Runner but succeeds in the Execute Anonymous window because the test method relies on existing data in the sandbox.

Isolation of Test Data: By default, Apex tests do not have access to pre-existing data in the organization. Tests are executed in an isolated context to ensure they are not dependent on external data, promoting reliability and repeatability.

"Apex test methods don’t have access to pre-existing data in the organization. The test methods have access to all data that they create and to changes made by other test methods (unless the @IsTest(Isolated=true) annotation is used)."— Apex Developer Guide: Isolation of Test Data from Organization Data in Unit Tests

Execute Anonymous Window: When code is run in the Execute Anonymous window, it executes in the current user's context and has access to all organization data, including existing records.

"All Apex code runs in system mode. The only exception is anonymous blocks, such as code executed using the Execute Anonymous window or the SOAP API."— Apex Developer Guide: Enforcing Object and Field Permissions

Solution: To fix the failing test, the developer should create the necessary test data within the test method or use the @IsTest(SeeAllData=true) annotation if access to existing data is absolutely necessary (though this is generally discouraged).

"If you specify @IsTest(SeeAllData=true), the test method has access to all data in the organization, but the test can be slower and less reliable because it might depend on data that changes."— Apex Developer Guide: Using the SeeAllData Annotation

Why Other Options Are Incorrect:

Option A: While calling an @future method requires special handling in tests, it would not cause the method to pass in Execute Anonymous and fail in tests.

Option C: A syntax error would prevent the code from compiling, so it wouldn't run in either context.

Option D: Using System.runAs is only necessary when testing code that behaves differently for users with different profiles or permissions.

Conclusion: The discrepancy arises because the test method is attempting to access data that isn't available in the test context, leading to its failure in the Test Runner but success in the Execute Anonymous window.

To ensure that the variables creditAmount and debitAmount cannot change once assigned, the final keyword should be used.

Option B: Use the final keyword and assign its value in the class constructor.

Declaring a variable as final allows it to be assigned once.

Assigning the value in the constructor is acceptable for instance variables.

Option D: Use the final keyword and assign its value when declaring the variable.

Assigning the value at declaration ensures it cannot be changed later.

Why Not Other Options:

Option A: Using static with assignment in the constructor is invalid because static variables belong to the class, not instances.

Option C: Using static and assigning in a static initializer doesn't prevent reassignment unless final is also used.

References:

Final Variables:

"You can declare instance variables as final. A final variable can only be assigned a value once, either when you declare it or in the constructor."— Apex Developer Guide: Classes, Objects, and Interfaces