What is commonly used to protect wood surfaces from moisture?

The correct response is the option that identifies a sealer as a common product for protecting wood surfaces from moisture. Sealers are specifically designed to penetrate the wood and create a barrier that prevents water from entering the wood fibers, which helps to prevent warping, swelling, and decay. This property makes sealers particularly effective for outdoor wood applications or in areas prone to humidity.

While paint and stain can also offer some level of moisture resistance, they primarily serve different purposes. Paint provides color and a durable surface finish, while stain enhances the natural grain of the wood. However, neither is as effective as a dedicated sealer in providing a moisture barrier. Glue, on the other hand, is an adhesive used for bonding materials together and does not serve a protective function for wood surfaces against moisture.