Which of these routes allows for substances to be absorbed through the skin?

The route that allows for substances to be absorbed through the skin is the transdermal route. Transdermal delivery involves applying a medication or substance onto the skin's surface, where it can penetrate the skin layers and enter the bloodstream. This method is commonly used with patches that apply medication continuously over a period of time, providing a consistent release of the substance without the need for frequent dosing.

Other routes listed, such as inhalation, injection, and oral, do not primarily involve absorption through the skin. Inhalation delivers substances directly to the lungs, injection involves introducing substances into the body via a needle, and oral administration entails consuming substances through the digestive system. Each of these routes has its own mechanisms of absorption and effects, but only transdermal specifically targets skin absorption.