Advanced Placement (AP) Computer Science 2025 – Comprehensive All-in-One Guide to Exam Success!

In an 'if' statement, a common action is to check the truth of a given condition. The primary purpose of an 'if' statement is to evaluate a condition and determine the flow of execution in a program based on whether that condition is true or false. When a condition evaluates to true, the code block associated with the 'if' statement is executed; otherwise, the code in that block is skipped. This mechanism enables decision-making in code, allowing for different actions to be taken depending on varying circumstances within the program.

The other actions listed do not align with the fundamental purpose of an 'if' statement. Defining new functions pertains to creating reusable blocks of code, while listing variables involves organizing data without any conditional logic. Declaring data types refers to specifying the type of data that can be stored in a variable, which is a separate structural aspect of programming. Thus, the essence of an 'if' statement is centered around condition evaluation, making the correct answer focused on checking the truth of a given condition.