Institution: জাতীয় কবি কাজী নজরুল ইসলাম বিশ্ববিদ্যালয় (JKKNIU) Exam: গ ইউনিট || জাতীয় কবি কাজী নজরুল ইসলাম বিশ্ববিদ্যালয় || 2012
Topic: Mathematics (সাধারণ গণিত), Subject: Mathematics (সাধারণ গণিত)

Best Answer Explained in Below-:

To determine which number among the options is not a prime number, we first need to understand what a prime number is. A prime number is defined as a natural number greater than 1 that has no positive divisors other than 1 and itself. This means that a prime number can only be divided evenly (without leaving a remainder) by 1 and the number itself.

Let’s evaluate the given numbers:

- **227**: The divisors are only 1 and 227, making it a prime number.
- **221**: This number can be divided by 1 and itself (221), but also by 13 and 17 (since 13 x 17 = 221). Therefore, 221 has divisors other than 1 and 221, indicating it is not a prime number.
- **223**: Similar to 227, the only divisors are 1 and 223, qualifying it as a prime number.
- **229**: It too has no divisors other than 1 and 229, thus it is a prime number.

Since 221 has factors other than 1 and itself, it is the only non-prime number among the options, making it the correct answer.