7
7 and/or 13
One pair: it's a prime number.
7
91 is the only composite factor of 91. 7 and 13 are prime.
The greatest common factor is 13. Both 26 and 91 are divisible by 13. The greatest common factor cannot be larger than the smallest number, so the greatest common factor in this case is the smallest number - 13, since it is a factor of the other two numbers.
A person unfamiliar with prime numbers could mistake 51, 91, or any other composite number for a prime number if the person did not factor the number to make sure it had no other factors then 1 and itself.
97 is prime.
The second prime number after 91 is 101.
97 is the next prime number after 91.
7 x 13 = 91
91 cannot be in the prime factorization of any number because it is not a prime number itself.
91 is not a prime number. 91 is a composite number having the factors 1, 7, 13, and 91.