Which of these ingredients is added to foundations to give them good coverage?

The inclusion of titanium oxide in foundations is recognized primarily for its ability to provide excellent coverage. Titanium dioxide is a naturally occurring mineral that is widely used in cosmetic formulations due to its opaque quality, which helps to effectively mask imperfections on the skin. This ingredient not only offers good pigmentation but also contributes to a matte finish, making it a popular choice in many makeup products.

Additionally, titanium dioxide has protective properties, as it acts as a physical sunscreen which shields the skin from ultraviolet radiation. This dual function enhances its value in both aesthetic and protective applications in foundation formulations.

In contrast, the other ingredients listed may serve different purposes in cosmetics but do not primarily fulfill the role of providing coverage. Aluminum hydroxide is mainly used as a thickening agent and stabilizer. Zinc oxide does provide some coverage and sun protection but is not as commonly used as titanium dioxide for achieving full-skin coverage in foundations. Silicon dioxide, while beneficial for texture and oil absorption, does not offer the same level of pigmentation needed for effective coverage in foundation products.