It would be D.
Whole numbers are numbers that are positive and don't have a decimal, which therefore, all are integers.
Integers are numbers that don't have a decimal, and are either negative or positive, so around half of them are whole numbers.
Um, all numbers are real, that's why we're talking about them.
Irrational numbers are numbers that can't be expressed as fractions. All integers can be expressed as fractions. Therefore, D is true.