What turns alkaline pyrogallol solution dark brown?A.{H_2}B.{H_2}OC.{O_2}D.None of the above

The correct answer is **C. \(O_2\) (oxygen)**.

### Explanation:
Alkaline pyrogallol is a solution of pyrogallol (also known as 1,2,3-trihydroxybenzene) in an alkaline medium. Pyrogallol has the ability to absorb oxygen from the air. When oxygen is present, it reacts with the alkaline pyrogallol solution, leading to oxidation.

This oxidation reaction causes the pyrogallol to turn into brown-colored oxidation products, such as quinones, giving the solution a dark brown color.

### Reaction:
\[
\text{Pyrogallol} + \ O_2 \longrightarrow \text{Oxidized products (dark brown)}
\]

Thus, it is the presence of **oxygen** (\(O_2\)) that causes the alkaline pyrogallol solution to turn dark brown. Neither hydrogen (\(H_2\)) nor water (\(H_2O\)) has this effect.