The real number system comprises of natural numbers, whole numbers, integers, rational numbers, and irrational numbers.
The third statement is true about the real number system. Natural numbers are numbers we use for counting or ordering i.e., numbers 1, 2, 3, and so on. These numbers, also called positive integers, can be expressed as a fraction where the denominator is not equal to 0. Rational numbers are numbers that can be expressed as a fraction where the numerator is an integer and the denominator is a natural number i.e., the form a/b where a and b are integers, and b is not equal to zero.
For example, you can express a natural number like 3 as 3/1. Therefore, it can be said that all natural numbers are indeed rational numbers.
As for the other options:
- Not all real numbers are irrational. Real numbers include both rational (those that can be expressed as a fraction) and irrational numbers (those that cannot be expressed as a fraction),
- Not all rational numbers are whole numbers. Rational numbers include fractions, decimals, and whole numbers,
- Similarly, not all integers are natural numbers, integers include both positive and negative numbers and zero, while natural numbers are only the positive numbers, excluding zero.