The necessary condition for a number to be rational is:
It must be able to be expressed as an integer divided by a nonzero integer.
So, the correct answer is: O It must be able to be expressed as an integer divided by a nonzero integer.
Rational numbers can be expressed as a fraction (which is the same as being expressed as an integer divided by a nonzero integer), but the fraction form is not the defining characteristic. A rational number can be written in fraction form, but not all fractions are explicitly written as such. For example, the integer 5 is rational because it can be expressed as \( \frac{5}{1} \).