One form of ___ is imprinted blotted paper.

The correct answer is linked to the method of administration commonly associated with the substance known as LSD (lysergic acid diethylamide). LSD is often distributed in a form that includes blotter paper, which is a type of absorbent paper that has been infused with a precise dose of the drug. This method of delivery allows for easy consumption, as users can simply place a square of the impregnated blotter paper on their tongue.

The other substances listed have different methods of distribution and consumption. Mescaline, for example, is primarily derived from cactus species and is typically consumed in its natural form or as a powder. Psilocybin is the active compound found in certain mushrooms, which are usually consumed whole, dried, or brewed into tea. PCP (phencyclidine), once used as an anesthetic, is often found in liquid form, as a powder, or as a 'wet' substance when applied to herbs or tobacco for smoking.

Understanding the specific forms that these substances take can help clarify why blotter paper is uniquely associated with LSD.