Which type of data is usually used to test the program's handling of unexpected conditions?

The selected answer is correct because boundary data specifically focuses on the values at the limits of acceptable input ranges. Testing with boundary data is important in software development as it helps developers identify how their program handles edge cases and unexpected conditions. These edge cases include the lowest and highest possible values that a variable can take, as well as values just inside and outside these limits.

By using boundary data, testers can uncover potential errors or bugs that occur when the input values are at the extremes of the allowable range. This type of testing is crucial for ensuring that the program behaves as expected in real-world scenarios, particularly when dealing with user inputs that could be erroneous or out of the designated range.

Normal data typically refers to typical inputs that the program is designed to handle, while extreme data represents tests that go well beyond the expected range, which may not directly test the handling of expected inputs. Dummy data might include irrelevant or placeholder data that doesn't scrutinize the program's response to actual boundary conditions. Therefore, boundary data is the most effective choice for testing a program's response to unexpected inputs.