A prime number is one that cannot be divided evenly by anything other than itself and one.
88 is not a prime number because it can be divided by 8, among many other numbers. 65 is not prime because it can be divided by 5, among other factors. 51 is not a prime number because it is divisible by 3, along with other numbers. 17 is not divisible by anything other than itself and one, so... 17 is the prime number.
Hope that helped you.
(Hint: Use divisibility rules to help you figure these types of problems out faster.)