Rational numbers are numbers that can be written as a fraction of two integers (as long as the denominator isn’t 0).
5 = 5/1, so it’s rational
1/2 and 1/3 are fractions, so they’re rational.
As decimals, rational numbers either stop or repeat.
1/2 = 0.5 (That stops.)
1/3 = 0.3333333… (That’s repeating.)
Of the two possible answers left, does one of them repeat? If so, it must be the other answer.