Which of the following can enhance the longevity of treated wood?

Applying multiple layers of preservative significantly enhances the longevity of treated wood. When additional layers of preservative are applied, they create a more robust barrier against moisture, insects, and fungi, all of which contribute to wood decay. This extra layer of protection helps ensure that the preservatives penetrate deeply and adequately fill the pores of the wood, thus enhancing the material's resistance to environmental factors that can lead to deterioration.

In contrast, using thicker wood cuts may provide some inherent durability due to the increased mass, but it does not specifically enhance the protective qualities of the wood treatment itself. Increasing exposure to sunlight can actually degrade some wood treatments and lead to faster breakdown of the wood fibers due to UV radiation. Reducing air exposure, while beneficial to some extent, does not provide as direct a benefit to wood preservation as applying multiple layers of a preservative, which actively works to repel water and protect wood from biological threats.