Final answer:
The NOW function returns the current date and time without requiring any arguments, making it the correct answer. TODAY is similar but only returns the date. Other options require at least one argument.
Step-by-step explanation:
The function which returns a valid value without any arguments is NOW. The NOW function is used to display the current date and time, and it does not require any arguments to be passed. Function TODAY also returns a valid value without any arguments, providing the current date only. The other options provided - SQRT, COUNT, and ROUND - all require arguments. SQRT needs a number to calculate its square root, COUNT requires a range of cells to count, and ROUND needs a number and the number of decimal places to which to round.