Answer:
I didnt really know how to put it into words, so I filled in your chart for you. Hope you like it and that I was not too late in completing this.
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.