Which type of variable can only assume two values: true or false?

A Boolean variable is specifically designed to represent binary values, meaning it can only hold one of two possible states: true or false. This characteristic makes it essential in programming for controlling flow and logic. For instance, Boolean variables are widely used in conditional statements and loops to dictate the execution path based on certain conditions.

In contrast, integer variables can represent a vast range of numerical values (positive, negative, and zero), while character variables hold individual letters, numbers, or symbols. String variables are used to manipulate sequences of characters, effectively representing text data. Thus, neither integer, character, nor string variables can be limited to just true or false values, which is what distinctly qualifies the Boolean variable as the correct answer.