What is applied to metal surfaces to remove the oxide film before soldering?

The correct choice, flux, is essential in the soldering process as it serves to clean metal surfaces by removing the oxide film that can hinder the formation of a proper solder joint. When metals naturally oxidize, they develop a thin layer of oxides on their surface, which can inhibit the adherence of the solder. Flux helps by chemically reacting with these oxides, allowing for better wetting of the solder to the metal.

Using flux ensures that the areas where the solder is applied are clean and free from contaminants, which is crucial for achieving a strong and reliable bond. Additionally, flux can help to lower the surface tension of the molten solder and improve flow, further aiding in the soldering process.

Other options, while they may seem relevant, do not fulfill the specific role that flux does in this context. Solder itself is the material that joins the metals, acid can sometimes be used for cleaning but is not the preferred method due to its aggressive nature, and paste is a general term that may refer to various substances but does not specifically indicate the cleaning function that flux provides prior to soldering.