A prime number is a number whose factors only include 1 and that number itself, examples of prime numbers would be 2 (1; 2), 5 (1; 5), and 11 (1; 11). In order to find the prime number, you must calculate all factors in each number. 63 has the following factors: 1; 3; 7; 9; 21; 63, so it isn't a prime number. 27 has the following factors: 1; 3; 9; 27, so it isn't a prime number. 33 has the following factors: 1; 3; 11; 33, so it also isn't a prime number. 7 only has two factors, 1 and 7, meaning that it is a prime number. Hence, this would be the correct answer.
