What is the main principle of test-driven development?

The main principle of test-driven development (TDD) is to convert requirements into test cases before actual software development begins. This approach emphasizes writing automated tests based on the requirements and expected behavior of the code before any implementation takes place. The process involves writing a test that defines a function or improvements in the code, which initially fails since the function hasn't been developed yet. Developers then proceed to write the minimum code necessary to pass the test, followed by refactoring the code for optimization.

This cycle of writing a test, developing the code, and then refactoring ensures that the software is continually tested against the defined requirements and specifications. It leads to better-designed, more reliable, and bug-free software as developers are encouraged to think critically about requirements and design before implementation, ensuring that the developed features meet the expected outcomes.

The other options do not align with the principles of TDD. Developing software without any prior testing does not ensure quality or correctness. Tracking software development only after completion isn't in line with TDD's proactive approach to quality assurance. Finally, testing software only when issues arise contradicts the fundamental TDD methodology, which integrates testing into the development process from the outset, thus promoting a more thorough and effective development cycle.