Final answer:
Ipratropium is the cholinoceptor blocking drug that is the least absorbed by the brain, due to its structure as a quaternary ammonium compound, limiting its ability to cross the blood-brain barrier.
Step-by-step explanation:
Out of the drugs listed, Ipratropium is the one that is the least absorbed by the brain. This is because ipratropium is a quaternary ammonium compound, which does not readily cross the blood-brain barrier. In contrast, scopolamine, benztropine, and atropine are tertiary amines that can cross the blood-brain barrier to a greater extent. Ipratropium's poor brain penetration makes it a suitable drug for respiratory treatments, such as in chronic obstructive pulmonary disease (COPD), because it minimizes systemic side effects.