(D) Tartaric acid
Tamarind contains tartaric acid, which is acidic in nature and turns blue litmus red.
Tamarind is a natural source of tartaric acid. The question tests knowledge of naturally occurring acids. Acetic acid is in vinegar, methanoic acid (formic acid) is in ant stings, and oxalic acid is in tomatoes — none are the primary acid in tamarind. Tartaric acid is the correct answer and is a standard fact expected in CBSE exams.