Concept:Lemon fruit is a rich source of citric acid, a weak organic acid.Explanation:Citric acid is a six-carbon tricarboxylic acid. It is mainly produced by fungal fermentation. Chemical synthesis is possible but more expensive than fermentation.Other acids are not the primary acid in lemons:- Malic acid is found in apples and contributes to their sour taste.- Tartaric acid occurs naturally in grapes and is used as cream of tartar in baking.- Acetic acid is the main component of vinegar.Only citric acid is the major acid in lemons.Answer:B. citric acid