The correct answer is option 3, Baking soda.

A mixture is a combination of two or more substances that are physically combined and can be separated using physical methods, such as filtration or distillation.

Toothpaste and toilet soap, options 1 and 2, are both examples of mixtures. Toothpaste is a mixture of several components, including abrasives, fluoride, flavorings, and detergents. Toilet soap is a mixture of fats or oils, water, and various additives such as fragrances and colorants.

Option 4, Vinegar, is also a mixture. Vinegar is made through the fermentation of ethanol by bacteria, resulting in acetic acid. It also contains water and trace amounts of other compounds.

However, option 3, Baking soda, is not a mixture. Baking soda, or sodium bicarbonate, is a pure compound. It is a crystalline solid with a specific chemical formula, NaHCO3. It is not composed of different substances physically combined but rather consists of a single chemical substance.

In summary, while toothpaste, toilet soap, and vinegar are examples of mixtures, baking soda is not.