Which material commonly used for hardboard siding is primed at the factory?

The choice of plywood as a material commonly used for hardboard siding that is primed at the factory is accurate. Plywood is composed of thin layers of wood veneer that are glued together, creating a durable and stable product. When used for siding, factory priming is typically applied to enhance adhesion for paint and improve resistance to moisture and environmental factors. This factory-primed surface allows for easier application of final coatings during installation and helps protect the plywood from the elements prior to the homeowner painting or finishing it.

In contrast, other materials mentioned, while used for siding, do not typically undergo the same priming process at the factory. Fiberglass is often used in different applications and may not be manufactured in a way that involves priming as hardboard siding does. Aluminum siding is prefinished rather than primed, typically coated with a factory-applied finish to create a weather-resistant layer. Vinyl siding comes pre-colored and does not require priming since it is manufactured in a variety of colors and does not need painting. Therefore, the choice of plywood for hardboard siding, which is commonly factory-primed, stands out as the correct answer in this context.