What type of automobile structure is welded and/or bonded into one unit?

The correct choice is unibody, which refers to a type of automobile construction where the body and frame are integrated into a single structure. This design offers several advantages, including increased strength and rigidity, improved safety through better energy absorption in a collision, and lighter weight which contributes to better fuel efficiency.

In a unibody design, various components such as the roof, floor, and side panels are welded or bonded together, creating a cohesive unit that can withstand stress and impacts effectively. This method of construction has become the standard in modern vehicle manufacturing due to its efficiency in production and performance benefits.

The other types listed, such as body-on-frame, modular, and chassis, do not describe this integrated approach. Body-on-frame construction involves a separate frame to which the body is attached, resulting in a different structural integrity and vehicle performance profile. Modular structures often refer to components that can be assembled in various configurations but do not constitute the monolithic construction characteristic of unibody designs. The chassis generally refers to the base frame of a vehicle, which supports the body and other components but is not a completed structure itself.