Which of the following numbers is NOT prime?

Correct option is C

Concept Used:

A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself.  

Solution:

Now, let's check each option:

A. 811: 811 is a prime number. Its only divisors are 1 and 811.

B. 317: 317 is a prime number. Its only divisors are 1 and 317.

C. 817: 817 is not a prime number. It is divisible by 19 and 43 (19 $$\times$$​ 43 = 817).

D. 313: 313 is a prime number. Its only divisors are 1 and 313.

Therefore, the number that is NOT prime is C. 817.