The correct answer is B. Frost.
Frost is a limiting factor in temperate and boreal forests because it can kill plants and trees. Frost occurs when the temperature drops below freezing, and it can cause damage to plants in a number of ways. For example, frost can cause leaves to wilt and turn brown, and it can also damage the stems and roots of plants. In severe cases, frost can kill entire plants.
Nutrients, oxygen, and sunlight are not limiting factors in temperate and boreal forests. Nutrients are essential for plant growth, but they are usually not in short supply in these forests. Oxygen is also essential for plant growth, but it is not a limiting factor because there is plenty of oxygen in the atmosphere. Sunlight is also essential for plant growth, but it is not a limiting factor because there is plenty of sunlight in these forests.
In conclusion, the limiting factor in temperate and boreal forests is B. Frost.