Blue-gray smoke comes from the exhaust of a vehicle during deceleration. Of the following, which is the LEAST likely cause?

Blue-gray smoke from the exhaust during deceleration typically indicates that the engine is burning oil. This can occur for a few specific reasons, which can be closely linked to the engine's ability to manage oil and its components.

Worn valve guides and broken valve seals are both directly related to the intake of engine oil into the combustion chamber. Worn valve guides can allow excess oil to seep into the cylinders, especially when the engine is under less load, such as during deceleration. Similarly, broken or worn valve seals can fail to keep oil from entering the combustion chamber, leading to oil burn and resulting in blue-gray smoke.

Clogged oil return passages can also cause oil to accumulate in areas where it shouldn't, potentially leading to increased oil burning. However, this would generally not be solely associated with deceleration.

In contrast, while worn piston rings can contribute to oil burning by allowing oil to bypass the rings during normal operation, the description of the issue occurring specifically during deceleration points to oil being introduced more from the top end of the engine rather than the bottom where piston rings are located. Therefore, they are less likely the primary cause of blue-gray smoke in this scenario. This makes worn piston rings the least likely cause among the options provided