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). Since integers can be expressed as fractions (for example, 5 can be written as 5/1), they are always rational.
Decimals can be rational or irrational (for example, 0.5 is rational, but π is not). Positive and negative numbers can also include both rational and irrational numbers. Thus, the correct answer is integers.