What does blue-gray smoke from a vehicle's tailpipe during deceleration most likely indicate?

Blue-gray smoke from a vehicle's tailpipe during deceleration is most commonly associated with oil entering the combustion chamber. This can happen for a couple of reasons, both of which are suggested in the correct choice.

Bad valve seals allow oil to seep into the combustion chamber when the engine is under load, and during deceleration, this effect can be particularly noticeable because of increased vacuum in the intake manifold. Similarly, clogged oil return passages can cause oil to build up in places where it shouldn't be, leading to a similar issue during deceleration when the engine's dynamics change.

Thus, both bad valve seals and clogged oil return passages can contribute to the presence of blue-gray smoke, making the correct answer the combination of these two possibilities.