What is the greatest common factor of 89?

Answer 1

In arithmetic, as well as in many cases in everyday English, "common" means
"same for both".

A "common factor" is a number that's a factor of both of two numbers.
A single number can't have a "common" factor

Just like I couldn't tell you "the difference between a potato".

Answer 2

You need at least two numbers to find a GCF.