Which best identifies the sentence fragment? a: i have not seen the neighbors for a few days. b: a house with no cars in the driveway. c: the package is still on the front porch today. d: i noticed a large package on my neighbor's front porch yesterday.

B is a fragment.  A fragment is any sentence that either doesn't have a subject, or that doesn't have a verb.

B has no verb.  The house(which is the subject) is doing nothing.

If you wanted to correct this sentence you would get something like:
A house with no cars in the driveway usually means that no one is home.