To solve this question, the user needs to know about Equivalence classes which are used in software testing to divide a set of test cases into groups or sets that can be run together. Each set of test cases represents a particular behavior or functionality of the system under test.

In this question, the text box on the form accepts numeric values in the range of 18 to 25. Therefore, the valid equivalence classes will be the range of values between 18 to 25, including 18 and 25. Any value outside of this range will be an invalid equivalence class.

Now, let's go through each option and identify the invalid equivalence class:

A. 17: This option is not a valid equivalence class because it falls outside of the valid range of values between 18 to 25.

B. 19: This option is a valid equivalence class because it falls within the valid range of values between 18 to 25.

C. 24: This option is a valid equivalence class because it falls within the valid range of values between 18 to 25.

D. 21: This option is a valid equivalence class because it falls within the valid range of values between 18 to 25.

Therefore, the invalid equivalence class is:

A. 17

The answer is: A.