Identify the odd one:
3, 5, 7, 11, 13, 17, 19, 21

The correct option is 21.

To identify the odd one out from the given series, let us analyze each number in the list:
3, 5, 7, 11, 13, 17, 19, 21

Let us check the properties of each number:

• 3 is a prime number (divisible only by 1 and 3).
• 5 is a prime number (divisible only by 1 and 5).
• 7 is a prime number (divisible only by 1 and 7).
• 11 is a prime number (divisible only by 1 and 11).
• 13 is a prime number (divisible only by 1 and 13).
• 17 is a prime number (divisible only by 1 and 17).
• 19 is a prime number (divisible only by 1 and 19).
• 21 is a composite number because it has factors other than 1 and itself:
$21=3\times 7$

Since 21 is the only composite number in a list of prime numbers, 21 is the odd one out.