Which of the following is an example of a prime number?

A prime number is defined as a natural number greater than 1 that has no positive divisors other than 1 and itself. This means that a prime number can only be divided evenly (without leaving a remainder) by 1 and the number itself.

In this case, 19 is a prime number because it can only be divided evenly by 1 and 19. There are no other numbers that divide into 19 completely without a remainder.

In contrast, the other numbers provided can be factored into smaller integers. For instance, 12 can be divided by 1, 2, 3, 4, 6, and 12; 15 can be divided by 1, 3, 5, and 15; and 20 can be divided by 1, 2, 4, 5, 10, and 20. Since they have divisors other than 1 and themselves, they do not meet the criteria for being prime numbers. Therefore, 19 stands out as the only prime number in the list provided.