When a mixture of an aldehyde and Fehling solution is boiled in hot...

Fehling's solution is an alkaline solution of copper(II) sulfate complexed with tartrate ions. When an aliphatic aldehyde is heated with Fehling's solution, the aldehyde is oxidized to the corresponding carboxylate anion, and the cupric ions (Cu^{2+}$) are reduced to cuprous oxide (Cu_2O$).

The chemical reaction is given by:
$R-CHO + 2Cu^{2+} + 5OH^{-} \rightarrow R-COO^{-} + Cu_2O \downarrow + 3H_2O$

The $Cu_2O$ (cuprous oxide) formed appears as a red precipitate.

Answer: Cuprous oxide