Termination codons and mutant sequences?

I'd say the answer you are after is likely B. If you terminate an amino acid chain early it stands to reason that you will produce something that just won't work. If you merely substitute a base and change a single amino acid, you may see little change in the function at all (these tend to be called polymorphisms rather than mutations since they don't adversely affect things), a minor change so it doesn't work quite as well but still does the job ok, or a major change which takes out the function entirely or changes it altogether (eg it may go from only being active in the presence of other factors, to being active all the time).