The correct answer is option 4: Snow cover. When it comes to the percentage of reflected radiation, or albedo, the highest value is observed with snow cover. Albedo refers to the percentage of radiation, such as sunlight, that is reflected by a surface.
Snow has a high ability to reflect radiation due to its white and reflective surface. This means that a large portion of the sunlight that falls on snow-covered areas is reflected back into the atmosphere. As a result, the percentage of reflected radiation is highest with snow cover.
In comparison, wet ground (option 1) and forests (option 3) have lower albedo values. The percentage of radiation reflected by these surfaces is lesser due to the absorption of radiation by the ground or vegetation. Thick clouds (option 2) also have a lower albedo as they tend to scatter and absorb a significant portion of the incoming radiation.
To summarize, snow cover has the highest percentage of reflected radiation among the given options due to its high albedo.