Multiple choice general knowledge science & technology Which of these is a prime number - 791 641 687 323 Reveal answer Fill a bubble to check yourself B Correct answer Explanation A prime number is only divisible by 1 and itself. 641 is prime (not divisible by 2, 3, 5, 7, 11, 13, 17, 19, or 23). 791 = 7×113, 687 = 3×229, and 323 = 17×19 are all composite.