Which of the following is a well known natural source of oxalic acid found in foods and plants?

Introduction / Context:
Many organic acids occur naturally in foods and plants, giving them characteristic sour tastes and influencing their nutritional properties. Oxalic acid is one such acid, known for forming insoluble salts with calcium and contributing to kidney stone formation when consumed in large amounts. Recognising natural sources of oxalic acid helps in food science and health related discussions. This question asks you to select a natural source that is correctly associated with oxalic acid, distinguishing it from sources of other common organic acids.

Given Data / Assumptions:

• Oxalic acid occurs naturally in certain vegetables and fruits.
• Sour milk contains lactic acid produced by bacterial fermentation.
• Ant stings contain formic acid, which causes the burning sensation.
• Tamarind is rich in tartaric acid, used in some culinary and industrial applications.
• Lemon and citrus fruits contain citric acid as the main organic acid.

Concept / Approach:
Oxalic acid is commonly found in vegetables such as spinach, rhubarb and tomatoes, as well as in some other plant sources. It can combine with calcium to form sparingly soluble calcium oxalate. Other listed food items are better known for different acids: sour milk for lactic acid, ant sting for formic acid, tamarind for tartaric acid and lemons for citric acid. The approach is to match oxalic acid with its characteristic food and plant sources, especially tomatoes and certain leafy vegetables, and not confuse it with the acids present in other familiar substances.

Step-by-Step Solution:
Step 1: Recall that oxalic acid is present in vegetables like spinach, tomatoes and rhubarb and that high intake can contribute to kidney stone risk in susceptible individuals. Step 2: Tomatoes are often mentioned in school chemistry as a natural source of oxalic acid, along with some leafy vegetables. Step 3: Sour milk is produced when lactose is fermented by bacteria into lactic acid, not oxalic acid. Step 4: Ant stings contain formic acid (methanoic acid), which causes pain and irritation at the sting site. Step 5: Tamarind pulp contains a high proportion of tartaric acid, which provides its sharp sour taste. Step 6: Lemon juice is rich in citric acid, the main acid in citrus fruits. Step 7: Therefore, the correct natural source of oxalic acid among the options is tomato and some leafy vegetables.

Verification / Alternative check:
Nutrition and food chemistry references list oxalic acid content for various vegetables. Spinach, beet greens, Swiss chard, rhubarb and tomatoes are consistently identified as high oxalate foods. In contrast, lactic acid is associated with fermented milk products, formic acid with insects such as ants and bees, tartaric acid with grapes and tamarind and citric acid with citrus fruits. These associations are used in both health advice and food processing. This independent information confirms that tomatoes and certain vegetables are indeed recognised natural sources of oxalic acid.

Why Other Options Are Wrong:
Option B, sour milk, is dominated by lactic acid, not oxalic acid. Option C, ant stings, involve formic acid as the main active acid. Option D, tamarind, is well known for its tartaric acid content rather than oxalic acid. Option E, lemon juice, is characterised by citric acid and is used as a standard example in acidity discussions. None of these are identified primarily as oxalic acid sources in basic chemistry teaching. Only option A correctly points to tomato and some leafy vegetables as natural sources of oxalic acid.

Common Pitfalls:
Students may confuse different organic acids because many foods taste sour and because names like tartaric, citric and oxalic sound similar. A useful memory aid is to pair each acid with a key natural source: citric with citrus fruits, tartaric with tamarind and grapes, lactic with milk products, formic with ant stings and oxalic with certain vegetables including tomatoes and spinach. Organising information this way helps avoid mixing them up in exam questions.

Final Answer:
A well known natural source of oxalic acid is Tomato and some leafy vegetables, which contain measurable amounts of this dicarboxylic acid.