Fehling's solution works on the principle of redox reaction which results in:

Explanation

Fehling's solution is specifically designed to test for aldehydes, which are oxidized to carboxylic acids during the reaction. In this process, the Cu²⁺ ions from the Fehling's solution are reduced to Cu₂O, which precipitates as a red solid. Therefore, the correct answer is the oxidation of aldehyde. The other options are incorrect as they either misinterpret the nature of the reaction (like reduction of aldehydes) or focus on the wrong substrate (ketones), which do not react under these conditions. While copper(II) is involved in the reaction, the key reaction occurring is the oxidation of the aldehyde itself.