The correct answer is option 1: Methanoic acid. Methanoic acid, also known as formic acid, is secreted by the leaves of the Nettle plant and is responsible for the painful stings it causes.

Option 2: Citric acid is a weak organic acid found in citrus fruits. Citric acid is not the acid responsible for the stinging sensation caused by Nettles.

Option 3: Tartaric acid is a crystalline organic acid found in grapes. It is commonly used in food and beverages. Tartaric acid is not the acid secreted by Nettle leaves that causes stings.

Option 4: Acetic acid is the main component of vinegar and is found in various foods. It is not the acid that causes the painful stings when in contact with Nettle leaves.

In conclusion, the correct acid secreted by Nettle leaves that causes painful stings is option 1: Methanoic acid, also known as formic acid.