Solution

• Citric acid – Lemon: This pair is correct. Citric acid is found in citrus fruits like lemons and oranges.
• Lactic acid - Sour milk: This pair is also correct. Lactic acid is present in sour milk and dairy products as a result of fermentation.
• Oxalic acid - Ant sting: This is incorrect. Oxalic acid is found in certain plants like spinach and rhubarb, not in ant stings. Ant stings actually contain formic acid (methanoic acid), which is responsible for the stinging sensation.
• Methanoic acid - Nettle sting: This pair is correct. Methanoic acid (formic acid) is found in nettle stings and is what causes the stinging and burning sensation when touched.