The set of numbers that is always rational is integers.
Integers are whole numbers that can be positive, negative, or zero, and they can be expressed as a fraction (e.g., every integer \( n \) can be written as \( \frac{n}{1} \)), which satisfies the definition of rational numbers.
In contrast:
- Negative numbers include both rational and irrational numbers.
- Positive numbers also include both rational and irrational numbers.
- Decimals can be rational (like 0.5) or irrational (like π).
Thus, the answer is integers.