How can you have a "common" divisor, greatest or otherwise, of a single number? The idea of a common divisor is that two or more numbers share a divisor -- which is to say, they have the divisor in common. Sort of like saying that a common trait of redheads is that they have red hair.
The greatest divisor of any number is itself, but I suspect that trivial fact of being decidedly unhelpful.
You are going to have to be a little clearer about what it is you want. If you have given us a textbook or worksheet question verbatim, then go back to your teacher and get an explanation. DO NOT leave or give up until you understand what is being said.
