ANS:B - surface energy.

The driving force for the sintering of a powder compact is surface energy. Sintering is a process used to form a solid mass from powdered materials by heating them below their melting point, causing the particles to bond together through diffusion and rearrangement processes. During sintering, the particles in the powder compact undergo various transformations to reduce the total surface area and energy of the system. Surface energy refers to the energy associated with the surface of a material. In a powder compact, the individual particles have a high surface energy due to their large surface area. During sintering, as the particles come into contact and bond with each other, the total surface area of the compact decreases, leading to a reduction in the overall surface energy of the system. This reduction in surface energy is the driving force for sintering. The particles rearrange and bond together to minimize their total surface area, resulting in the formation of a solid mass with improved mechanical properties. Therefore, surface energy is the driving force that promotes the sintering process in powder compacts.