Explanation

Unit testing is the practice of testing small, isolated, and self-contained pieces of code, such as a single function or method. Databricks defines unit testing as an approach to testing self-contained units of code early and often. Verifying the logic of a specific mathematical routine in isolation is a classic example of a unit test.

Why the other options are incorrect:

• Manual testing involves human execution and observation without automated assertions; the scenario describes a line of code (automated).
• Functional testing validates an application's behavior against user-facing requirements, typically involving multiple units working together.
• Integration testing focuses on the interaction between different components or systems (e.g., how a function interacts with a database), rather than a numeric routine in isolation.
• End-to-end testing validates the entire data pipeline or application workflow from input to output, which is much broader than testing a single algorithm.