Which would be most appropriate when installing downslope side chimney flashing in an area with regular freeze-thaw cycles and accumulations of ice and snow?

The most appropriate choice for installing downslope side chimney flashing in areas with regular freeze-thaw cycles and accumulations of ice and snow is to use apron flashings over a water and ice dam protection membrane on the downslope side. In regions susceptible to freezing temperatures, adequate waterproofing is essential to prevent ice dams and subsequent water infiltration.

By using an apron flashing in conjunction with a water and ice dam protection membrane, it creates a robust barrier against moisture. The apron flashing directs water away from the chimney and ensures that any melting snow or ice that runs down the roof does not enter the areas around the chimney where water could potentially accumulate.

Additionally, a water and ice dam membrane provides an extra layer of safeguarding against leaks, particularly in the eaves and valleys where water may pool, which is a common problem when freeze-thaw conditions exist. This is crucial for maintaining the longevity of the roofing system and preventing damage to the structure. The other options do not provide the level of protection needed in such climates, making them less effective and more prone to failure.