The set of numbers that is always rational is integers.
Rational numbers are those that can be expressed as a fraction where both the numerator and denominator are integers (with the denominator not equal to zero). Integers are whole numbers that can be positive, negative, or zero, and they can always be written as a fraction (for example, the integer 3 can be expressed as \( \frac{3}{1} \)).
On the other hand, positive numbers and negative numbers include non-integer values (like fractions or decimals) which may or may not be rational. Decimals can also be irrational if they are non-terminating and non-repeating.
Therefore, the correct answer is integers.