Which of the following statements is true regarding expression functions?

The statement that functions may allow recursive calls is true because in Appian, expression functions can invoke themselves as part of a computation. This is particularly useful for solving problems that can be defined in terms of smaller subproblems, enabling a clear and effective way to perform tasks like calculating factorials or processing tree structures. Recursion allows for elegant solutions to problems where the logic can be naturally expressed in terms of itself.

Other options suggest limitations or inaccuracies about how functions operate in Appian. Functions do not require only manual input; they can take various types of inputs, including those from other expressions or variables. While certain functions may handle array data types, not every function is universally designed to do so. Lastly, it is incorrect to state that functions cannot return values, as this is a fundamental characteristic of functions in Appian, where they are defined to compute and return results.