Galvanic corrosion is likely to be most rapid when:

Galvanic corrosion occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte. The metal that is anodic (more corrosion-prone) will corrode faster than it would on its own, while the cathodic (less corrosion-prone) metal will be protected.

The correct choice relates to the surface areas of the metals involved. When the anodic metal has a smaller surface area compared to the cathodic metal, it experiences more rapid corrosion. This is due to the fact that the current flow generated in the galvanic cell is concentrated over a smaller area, leading to accelerated corrosion of the anodic metal. The smaller surface area of the anodic metal cannot dissipate the electrochemical reaction quickly enough, resulting in a higher rate of metal loss.

In contrast, a larger surface area on the anodic metal would result in a slower corrosion rate per unit area because the electrochemical reaction would be distributed over a wider surface. Similarly, if both metals had equal surface areas, the galvanic action would be minimal since the corrosion rate would be similar. Thus, having a smaller surface area for the anodic metal leads to the most rapid galvanic corrosion.