1. A. Doug lists all the reasons for killing Ralph Underhill, yet which reason does he say is

the most potent?

Ralph never supported the friendship. Ralph never cared for Doug as much as Doug cared for him.

Explanation:

What else? Is that the list?

"No. One final thing, more terrible than all the rest.

In all the years you went to Ralph's house to toss up small bits of gravel on his Fourth of July six-in-the-morning fresh dewy window or to call him forth for the arrival of dawn circuses in the cold fresh blue railroad stations in late June or late August, in all those years, never once did Ralph run to your house. Never once in all the years did he, or anyone else, prove their friendship by coming by. The door never knocked. The window of your bedroom never faintly clattered and belled with a high-tossed confetti of small dusts and rocks. And you always knew that the day you stopped going to Ralph's house, calling up in the morn, that would be the day your friendship ended."