The answer is sentence D. "If the fountain gushed at my very doorstep, I would not stoop to bathe my lips in it; no, though its delirium were for years instead of moments."
By saying this, Dr. Heidegger is explaining how he would refuse to drink from the Fountain of Youth even if its source were right where he lives. The reason behind this wise decision is that it causes "delirium," or madness, in the people who experience its rejuvenating effects; and Dr. Heidegger has witnessed it. Indeed, in the story, the characters who drink the fountain's elixir repeat their past mistakes (gambling, drinking, flirting, etc.) and get addicted to it.
