Based on my knowledge, E.B. White feels the "chill of death" in his visit to the Maine Lake because he realizes his own mortality and the fleeting nature of life. In his essay "Once More to the Lake," White reflects on his experiences visiting the lake as a child and returning with his own son. He describes how the lake looks and feels the same, but he himself has aged and is no longer the same person he was when he first visited.
As White watches his son prepare to jump into the lake, he is struck by the realization that he too will one day die, and that the lake will continue to exist long after he is gone. The coldness of the lake water is a physical reminder of the impermanence of life, and the "chill of death" is a metaphorical expression of the anxiety and discomfort he feels in confronting his own mortality.
So, the correct answer is iv. He realises his own death.