The main constituent of vinegar is acetic acid.

Option 1, citric acid, is incorrect. Citric acid is commonly found in citrus fruits like lemons and oranges, but it is not the main constituent of vinegar.

Option 3, ascorbic acid, is also incorrect. Ascorbic acid, also known as vitamin C, is found in foods like fruits and vegetables, but it is not the main constituent of vinegar.

Option 4, tartaric acid, is not the correct answer either. Tartaric acid is found in grapes and is commonly used in winemaking, but it is not the main constituent of vinegar.

The correct answer, option 2, is acetic acid. Acetic acid is the primary component of vinegar, giving it its characteristic sour taste and pungent smell. Vinegar is made through the fermentation process of ethanol by acetic acid bacteria.