Which of the following is the route of administration of a drug that is placed under the tongue?

The route of administration for a drug that is placed under the tongue is known as sublingual. This method allows for the rapid absorption of medication directly into the bloodstream through the tissues beneath the tongue. Sublingual administration bypasses the digestive system and first-pass metabolism by the liver, which can enhance the drug's efficacy and speed up its onset of action.

In contrast, intravenous administration involves injecting a drug directly into a vein, providing immediate effects; intradermal refers to injecting a drug into the dermal layer of the skin; and transdermal involves administering medication through the skin, typically via patches, for systemic effects over a longer period. Each of these routes serves different therapeutic purposes and has varying rates of absorption and effectiveness, which is why sublingual is the correct choice for the method described in the question.