Answer:
b. one that might have affected the outcome of a case.
Explanation:
A reversible error is an error of sufficient gravity to warrant reversal of a judgment on appeal. It is an error by the trier of law (judge), or the trier of fact (the jury, or the judge if it is a bench trial), or malfeasance by one of the trying attorneys, which results in an unfair trial. It is to be distinguished from harmless errors which do not rise to a level which brings the validity of the judgment into question and thus do not lead to a reversal upon appeal.
