Answer: B. 1
Step-by-step explanation: A common factor is something that two or more numbers have in common. It is 1 because you multiply 1 by 1 to get 1, and 1 by 2 to get 2.
(1 by 1) = 1
(2 by 1) = 2
These two numbers have the number 1 in common because both 1 and 2 can only be multiplied by 1 and itself to get the number. (That is also what you call prime numbers.)