(C) tartaric acid
Tamarind contains tartaric acid, which is acidic in nature and turns blue litmus red.
This is a direct factual question. Tamarind's sour taste is due to tartaric acid. Remember: acids turn blue litmus red. The other options — methanoic acid (ants/nettles), acetic acid (vinegar), oxalic acid (tomatoes/spinach) — are acids found in different sources.