The GCF of

is 7, so the factored expression is:

The GCF of:

is 3
The third expression doesn't have a common factor. 7 and 3 are not a multiple of the other, so this expression cannot be factored.
The GCF of the last one is:

is 7.
The third expression is the one that doesn't belong with the other three, because that one cannot be factored.