Four options have been given out of which three are alike in some manner, while one is different. Choose the odd one.

The logic followed here is:

All the numbers except option (3), the given numbers are the multiple of 11. 

Option1: 2321 → Multiple of 11

Option2: 825 → Multiple of 11

Option3: 1521 → Not a multiple of 11

Option4: 968 → Multiple of 11

Therefore, 1521 is the odd one option.

Important Points

• The priority will always be the prime number in the exams.
• There are some standard rules which need to be followed while selecting the priority.
• priority lists to be followed in exams according to their preferences are:

1. Prime number.
2. Square.
3. Cube.
4. Multiplication and divisions
5. Addition and subtraction.

• A prime number can be divided, without a remainder, only by itself and by 1.
• Some facts about the Prime numbers:

1. The only even prime number is 2. All other even numbers can be divided by 2.
2. If the sum of a number's digits is a multiple of 3, that number can be divided by 3.
3. No prime number greater than 5 ends in a 5. Any number greater than 5 that ends in a 5 can be divided by 5.
4. Zero and 1 are not considered prime numbers.
5. Except for 0 and 1, a number is either a prime number or a composite number. A composite number is defined as any number, greater than 1, that is not prime.