The set of numbers that is always rational is integers.
Rational numbers are defined as numbers that can be expressed as a fraction or ratio of two integers (where the denominator is not zero). Integers (which include positive numbers, negative numbers, and zero) fit this definition because any integer can be expressed as a fraction with a denominator of 1 (e.g., 5 can be written as 5/1). Thus, all integers are rational numbers.
The other options (positive numbers, negative numbers, and decimals) can include irrational numbers (for example, √2 is a positive irrational number), so they are not guaranteed to be rational.