The best explanation for the difference in weight observed by Mrs. Franklin's class is:
C. There was a gas given off during the reaction that was not weighed.
When vinegar (acetic acid) reacts with baking soda (sodium bicarbonate), carbon dioxide gas is produced as a byproduct of the reaction. This gas escapes into the air and is not captured in the weight measurement of the mixture after the reaction, leading to the observed decrease in weight.