He meant that what do you take in from looking at something. So if you are absently watching a tree and don't do anything else, you might miss the fact that the leaves are moving with the wind, but they are held fast by the connection they make with the branch that holds them.
Think carefully about what that actually means. When fall comes, the branch lets go and the leaves float to the ground.
Even if you think there is a temperature mechanism that loosens the hold the branch has on the leaves, Someone or Something put it there so it could happen.
You excuse me if I let my religion show somewhat. I do think it is some sort of miracle that lets something like that happens. And Thoreau wouldn't disagree. Those Transcendental writers (like Thoreau) all saw miracles in the ordinary.
The correct answer is The alligator slipped quietly into the water.
The first sentence does not agree since alligators should be a singular form, while the second and third are nonsensical because of lack of information.
