Which is an example of a valid function in a formula?

The choice of "Average(x, y, z)" as a valid function in a formula is correct because it follows the syntax for functions in Quickbase. The Average function is specifically designed to calculate the mean of a specified set of values. In this case, it takes three parameters: x, y, and z, which can represent field references or numerical values. Properly structured functions typically contain the function name followed by the arguments in parentheses, which is exactly what this option provides.

In contrast, "NAME?" is not structured as a recognized function, rather it seems like a question or incomplete reference. The option "[Due Date]" refers to a field but does not represent a function. It simply indicates a reference to a specific field in a Quickbase table. Lastly, "(3+x-4)" presents an expression but does not utilize any predefined functions, making it valid as an expression but not as a function in the context specified by the question. Thus, average is the only complete and syntactically correct function listed.