The given options are 31, 41, 51, and 61, and we need to identify the odd one out.

Option 1, 31, is a prime number as it can only be divided by 1 and itself.

Option 2, 41, is also a prime number.

Option 4, 61, is also a prime number.

Option 3, 51, is not a prime number as it can be divided evenly by numbers other than 1 and itself. Specifically, it can be divided by 3, 17, and 51. Therefore, option 3, 51, is the odd one out because it is the only number that is not prime.

To summarize, the odd one out is option 3, 51, because it is the only number among the given options that is not a prime number.