To answer this question, you need to understand the concept of equivalence classes.
Equivalence classes refer to a set of input values that are considered equivalent or produce the same result. In this case, the valid equivalence class for the numeric values in the text box is between 18 and 25.
Let's go through each option to determine which one is an invalid equivalence class:
Option A) 17 - This option is invalid because it falls outside the specified range of 18 to 25.
Option B) 19 - This option is valid because it falls within the specified range of 18 to 25.
Option C) 24 - This option is valid because it falls within the specified range of 18 to 25.
Option D) 21 - This option is valid because it falls within the specified range of 18 to 25.
Therefore, the invalid equivalence class is option A) 17 because it is outside the range of 18 to 25.