Which solution is used to test for aldehydes and produces a red precipitate?

Fehling's solution is specifically designed to test for aldehydes and is known for its ability to produce a red precipitate of copper(I) oxide (Cu2O) when an aldehyde is present. This occurs because aldehydes can reduce the blue copper(II) ions in Fehling's solution to copper(I) ions, leading to the formation of the red solid.

Benedict's solution also tests for reducing sugars and aldehydes, producing a range of colors from green to reddish-brown, depending on the concentration, but it is typically less definitive than Fehling's regarding the formation of a distinct red precipitate.

Manganese dioxide does not serve as a reagent for aldehydes and is more commonly associated with oxidation reactions rather than reduction tests.

Silver nitrate solution can react with aldehydes, producing a silver mirror in a tarnished test tube (Tollens' test), but it does not produce a red precipitate. Instead, it leads to the deposition of metallic silver.

The specificity and definitive red precipitate formation make Fehling's solution the correct choice for aldehyde detection.