Portability testing is concerned with how well software can be transferred from one environment to another. In the context of a bank using COTS (Commercial Off-The-Shelf) software, the sub-characteristic of replaceability becomes particularly relevant. This is because the bank does not want to create a dependency on any external COTS supplier, meaning it should be able to replace the software with another product without significant effort or operational disruption. Replaceability ensures that if needed, the bank can switch to different software, thereby mitigating the risk of supplier dependency.

Keyword-driven test automation is a framework where test cases are written using keywords that represent the actions or tests to be performed on the system. This is an extension of data-driven test automation, which focuses on separating test scripts from the test data, allowing the same test script to be run with various sets of data. Keyword-driven test automation further abstracts the process by allowing tests to be written in a more human-readable form that corresponds to business processes. This approach can improve maintainability and readability of test cases, making them easier to understand and modify. It's not necessarily the case that one is more maintainable or easier to develop than the other (Options C and D); rather, they serve different purposes in test automation strategy.

The scenario written in correct Gherkin format and appropriate for the User Story described is Option D:
* Given that three Purchase Orders with a total value of £21.75 have been placed so far this month,
* When the Purchasing Manager requests the list of Purchase Orders placed this month,
* Then the 3 orders and their total value of £21.75 are displayed.
This scenario correctly uses the Gherkin syntax with Given-When-Then structures that clearly specify the context, the action taken by the user, and the expected outcome, making it directly relevant to the user story's intent .

Model-based testing tools support the creation and execution of tests based on models of the system under test. Finite state machines (FSMs) are often used in model-based testing to describe the expected behavior of a system during execution. FSMs help in defining the states of the system and the transitions between these states based on events, which can then be used to generate test cases that validate the system's behavior against the model.

The correct description of continuous testing is that each new build of the system triggers a pre-defined set of tests to be executed automatically. This approach ensures that any changes or additions to the system are immediately validated, helping to catch issues early and streamline the development process .