Final answer:
Surface hoar is the correct answer; it forms on snow surfaces under clear, calm conditions with a significant temperature gradient, through sublimation.
Step-by-step explanation:
The correct answer to the question about the type of weak snow layer that forms from frozen dew deposition when air temperature is warm and the snowpack is cold with little wind is a. Surface hoar.
Surface hoar is a type of frost that forms on snow surfaces when conditions are clear and calm, and there's a sufficient temperature gradient. This happens when water vapor goes directly from the gaseous to the solid state, forming feathery crystals on the snow.
Though similar in appearance to dew, which forms when a surface is colder than the dew point of the surrounding air and water vapor condenses, surface hoar forms through sublimation. Surface hoar can become a weak layer if it gets buried by subsequent snowfalls, which can then lead to an increased risk of avalanche if the snowpack is disturbed.