To protect against _____ in aluminum siding, many manufacturers offer a thin backer board of insulation that fits behind each panel.

The correct answer pertains to the protection against dents in aluminum siding. Aluminum is lightweight and has a certain degree of malleability, which means it can be susceptible to physical impacts that may cause it to dent easily. Manufacturers provide a thin backer board of insulation to serve as a protective layer behind the panels, which can absorb some of the impact and help maintain the appearance and integrity of the siding.

While dents are the focus here, it's important to note that the backer board does not directly address issues like rusting or punctures associated with aluminum siding, as aluminum is naturally resistant to rust due to the formation of a protective oxide layer. Moreover, while warping can occur due to thermal expansion or other environmental factors, it is not specifically mitigated by the use of backer boards designed primarily for protection against dings and dents. This context emphasizes the targeted purpose of the backer board as a safeguard against common physical damage to aluminum siding.