The correct answer is option 3, Oxalic acid. Oxalic acid is predominantly found in tomatoes. Let`s dissect each option to understand why it may or may not be the correct answer.

Option 1, Acetic acid, is primarily found in vinegar and not in tomatoes. Therefore, this option is incorrect.

Option 2, Tartaric acid, is commonly found in grapes and wine, but it is not the predominant acid in tomatoes. Hence, this option is also incorrect.

Option 3, Oxalic acid, is indeed predominantly found in tomatoes. Oxalic acid is a naturally occurring acid that is found in many fruits and vegetables, including tomatoes. It is also present in small amounts in other foods such as spinach and rhubarb.

Option 4, Lactic acid, is commonly found in fermented foods like yogurt and sauerkraut, but it is not the primary acid found in tomatoes. Therefore, this option is incorrect.

In conclusion, the correct answer is option 3, Oxalic acid, as it is mainly found in tomatoes.