In programming, which of the following would most likely be used to validate user input in a TextBox?

Validating user input in a TextBox is typically accomplished through the use of a Boolean expression. A Boolean expression evaluates conditions and returns a true or false value based on the input data. By using these expressions, programmers can set specific criteria that the input must meet—for instance, checking if a string is not empty, ensuring that the input is numeric, or validating an email format.

For example, if you want to ensure a user enters an age that is a valid number, a Boolean expression can check whether the input string can be parsed into an integer and falls within a specified range. This direct evaluation of conditions makes Boolean expressions particularly suitable for input validation tasks.

Other programmatic components such as event handlers may be part of the overall input handling process, but they primarily respond to events like button clicks or changes in input rather than directly validating input. TextBox formatters are used for controlling how data appears but do not inherently enforce validation. Declaring variables is a part of writing code but does not contribute to the validation of user input itself.