Is it true that any fraction is in its simplest form if and only if the only common factor of the numerator and denominator is 1 or -1.

Yes

Step-by-step explanation:

When the greatest common factor of two numbers is 1 or -1, the numbers are known as relatively prime. This means there are no numbers that can be factored out of both of the numbers, meaning if the numerator and denominator are relatively prime, they can not be simplified any further.

12/11c

or

12

c

11