In "Once More to the Lake," what does White mean when he says "now the choice was narrowed down to two"?
The answer to your question is found in this passage from the essay as White tells of taking his son to the farmhouse close to the fishing camp for dinner:
Up to the farmhouse to dinner through the teeming, dusty field, the road under our sneakers was only a two-track road. The middle track was missing, the one with the marks of the hooves and the splotches of dried, flaky manure. There had always been three tracks to choose from in choosing which track to walk in; now the choice was narrowed down to two. For a moment I missed terribly the middle alternative.
White noticed the road they walked to the farmhouse consisted of only two tracks, the suggestion being those left by automobiles. A third track, the one in the middle between the two remaining ones, had once been there, also. We can infer it was made by those on horseback or by the horse that pulled a wagon or cart. This missing third track is one White remembers from his childhood when he could choose among three tracks in the dirt to decide where to walk. Going back to the camp as an adult, he finds that the road has changed, and he misses the way it once had been.