Option C is the correct answer because “most” is used to express something which is greatest in amount or degree, which is correct here because it follows “the” which is used to point on a particular clause here, on water being the most precious.
Option A is incorrect because although “very” has the correct meaning to substitute the blank, which means in a high degree, but it is incorrect because it cannot follow the, as “the” is used in this statement to emphasize the amount or degree to which something is affected, hence incorrect.
Option B is incorrect because “much” is usually used to express something to a great extent which signifies a quantity, it would be correct if it was “.. very much precious”, hence incorrect.
Option D is incorrect because “too” means in addition to, hence incorrect here.