Identify which condition is necessary for a number to be rational

A number is rational if it can be expressed as a quotient or fraction of two integers, where the denominator is not zero.

Therefore, the condition necessary for a number to be rational is that it can be written in the form a/b, where "a" and "b" are integers and "b" is not equal to zero.

The necessary condition for a number to be rational is that it can be written as a fraction of two integers, where the denominator is not zero. In other words, a number is rational if it can be expressed as a quotient or ratio of two integers.