Continuous Integration (CI) primarily aims to detect defects and issues early in the development lifecycle by integrating code changes frequently. By doing this, it ensures that software quality is maintained and potential integration issues are identified at an earlier stage, making them quicker and often less costly to resolve. This speeds up software delivery and ensures a more stable and reliable codebase.

The primary goal of functional testing is to validate that each function of a software application operates in conformance with the requirement specification. For mobile apps, this ensures that all features work correctly and as intended, covering all scenarios including failures and edge cases.

Incremental Integration Testing is a process where individual units or components of a software are integrated gradually to ensure that interface defects are detected and corrected. It helps in detecting issues that can arise during component interaction.

Adapting to changes in the application under test is a challenge as any modifications in the application's UI or functionality can break existing automated test scripts. This requires regular updates to scripts, making maintenance resource-intensive.

The "Test Plan" is a comprehensive document that outlines the scope, approach, resources, and schedule of testing activities. It serves as a blueprint for all testing processes and activities, ensuring that every aspect of the testing process is covered and understood by all stakeholders.

"Photoshop" is a graphic design and photo editing software and is not typically used in the risk identification process in software testing. On the other hand, Brainstorming, Risk Matrices, and Flowcharting are common techniques and tools used to identify and visually represent risks.

In the Top-down approach of incremental integration testing, testing starts from the top or main module and moves downwards, incorporating sub-modules one by one. This approach allows testing of the main functionality earlier in the development process, even if all sub-modules aren't complete yet.

The Error Rate in usability testing refers to the number of errors users make when interacting with a product or system. A high error rate may indicate that the user interface is not intuitive or is confusing, necessitating design changes to enhance usability.

The principle that "Exhaustive Testing is Impossible" acknowledges that it's impractical to test every possible scenario due to constraints like time. In cases where time is limited, prioritizing test cases becomes crucial, focusing on high-risk or high-importance areas of the application.

The principle of "Keyboard Accessibility" ensures that all features and functionalities on a website or application can be accessed and executed using only the keyboard. This is essential for users who cannot use a mouse or other pointing devices effectively.