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.
This means that a rational number can be written in the form \( \frac{a}{b} \), where \( a \) is an integer and \( b \) is a nonzero integer. Rational numbers can include both positive and negative values, as well as whole numbers and fractions. The other options provided are either incorrect or not necessary conditions.