Explanation:

Toothpastes contain various ingredients, each serving a specific purpose, including appearance. \nOption A: 'Mica' is often added to toothpaste to provide a shimmering or sparkly effect, enhancing its aesthetic appeal. However, it is not the primary agent responsible for making toothpaste white. \nOption B: 'Fluoride' (e.g., sodium fluoride, stannous fluoride) is a crucial active ingredient in most toothpastes, used for strengthening tooth enamel and preventing cavities. It does not contribute to the white color of the toothpaste itself. \nOption C: 'Titanium oxide' (specifically titanium dioxide, $\text{TiO}_2$) is a widely used white pigment in many products, including paints, plastics, and cosmetics. It is commonly added to toothpaste to give it its characteristic opaque white color. Therefore, this option is correct. \nOption D: 'Silica' (e.g., hydrated silica) is primarily used as an abrasive agent in toothpaste to help remove plaque and stains from teeth, and also as a thickener. While it is a white powder, its main function is not to impart the white color to the entire product as a pigment. \nThus, titanium oxide is the ingredient primarily responsible for the white color of toothpaste.