Find the odd number / letters / word from the given alternative:

13
17
23
37 E. 73

The odd number in the given list is 13. All other numbers are prime numbers. A prime number is a natural number greater than 1 that is not a product of two smaller natural numbers. A natural number greater than 1 that is not prime is called a composite number.

13 is not a prime number because it is divisible by 1 and 13.

17, 23, 37, and 73 are all prime numbers because they are not divisible by any number other than 1 and themselves.