The set of numbers that is always rational is integers.
Rational numbers are defined as numbers that can be expressed as the quotient or fraction of two integers, where the denominator is not zero. Integers (whole numbers including positive numbers, negative numbers, and zero) can all be expressed as fractions (for example, an integer \( n \) can be written as \( \frac{n}{1} \)).
In contrast, while decimals can be rational (like 0.5) or irrational (like 0.333... or \(\sqrt{2}\)), the set of negative numbers includes both rational (like -3) and irrational (like -\(\sqrt{2}\)) numbers. Positive numbers can also be either rational (like 2) or irrational (like \(\pi\)).
So the correct and always rational set is integers.