. . . . . . . . water is . . . . . . . . elixir of life, but . . . . . . . . water of this pond is poisonous.

Use of article is omitted before names of substances and uncountable noun. 'Water' is an uncountable noun. Hence, no article is used before this word. Of the options given, A and B are incorrect as they mention the use of the article, 'the' before water.
The noun, 'elixir' means 'a magical or medicinal potion'. The definite article, 'the' is used before names of things which are unique/ one of their kind. When we say, 'Water is the elixir of life', the article, 'the' emphasizes that 'water is the only elixir of life'. Hence, the article 'the' must precede 'elixir'. Option D is incorrect as it suggests the use of the indefinite article, 'an' before 'elixir.
The definite article 'the' is also used we talk about a particular person or thing. 'Water of this pond' refers to a particular noun. Hence, the article, 'the' should be used before the phrase, 'water of this pond'.