Which of the following numbers is 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. In other words, a prime number can only be divided evenly by 1 and the number itself, making it impossible to break it down into other factors.

In this case, 29 is identified as a prime number because it can only be divided evenly by 1 and 29. No other numbers can multiply together to make 29, confirming its status as a prime.

In contrast, the other numbers listed are not prime. For example, 21 can be factored into 3 and 7, which means it has divisors other than 1 and itself. Similarly, 25 can be factored into 5 and 5, indicating it has more divisors. Lastly, 35 can be factored into 5 and 7, also demonstrating that it is not a prime number. This distinction emphasizes the unique characteristics of prime numbers, particularly their indivisibility by any other numbers apart from 1 and the number itself.