A drug is developed that, due to its molecular shape, blocks the action of substance

a. which shape would the drug molecule most likely resemble?

It should have the shape number 2.

The substance A has a region that will bind to the nerve cell Y.

The drug developed that's going to block the substance A, will have to have a region that is complementary to the substance A. If those two regions get linked the substance A will have nowhere else to bind.