What combination of exposure factors would most likely produce the shortest-scale contrast?

The combination of exposure factors that would most likely produce the shortest-scale contrast involves a higher kilovoltage (kV), which increases the energy of the x-rays produced, thus reducing the overall contrast in the resulting image. Higher kV settings increase penetration and scatter, leading to a narrower difference between light and dark areas, which is characterized as short-scale contrast.

In the scenario of option D, the kV is set at 80, which is relatively moderate, but since it's paired with an appropriate mAs of 20 and a grid ratio of 10:1, it can help achieve lower contrast in the resulting image. The grid ratio plays a role in reducing scatter, but with a higher mAs and this kV setting, the net effect tends to favor lower contrast production.

In contrast, the other options present higher kV settings (90 kV), and when paired with their respective mAs values and grid ratios, may lead to higher overall image contrast rather than mimicking short-scale characteristics due to the inherent higher energy x-rays they represent. Thus, option D stands out as the most likely to produce the shortest-scale contrast, given the balance of exposure factors it offers.