ANSWER
D. rational numbers
Step-by-step explanation
A hat size can't be integers, because that includes negative numbers. It could be whole numbers, but there are sizes that aren't just whole numbers, but they have decimals or fractions.
For example, the smallest size of a hat usually is 6 3/4, which is definitely not irrational - because there's a fraction. Therefore, the set that is more reasonable to describe a person's hat size is rational numbers.