A program that Thomas has written is complete. It has been tested thoroughly, and some outputs were not as expected. Thomas is now going through the code carefully attempting to establish the cause of these errors and change the code to remove them. What is this procedure called?

Debugging.

Explanation:

Debugging is a common process in the world of programming. It's done to find out cause of errors, and correct them after finding them. Although finding errors and correcting them may make more errors popup, but this is something common in software development.

Most IDEs (Integrated development environment) provide great tools for debugging like pause execution, move to the next statement, break loop etc.