Final answer:
The term tundra is best defined as C. treeless plain, as it is a biome characterized by its cold climate and lack of trees, and is described as such in the context of the provided schoolwork question.
Step-by-step explanation:
The term tundra refers to a biome characterized by its cold climate, limited precipitation, poor nutrients, short growing seasons, and lack of trees. In the context of the question, the tundra is specifically described as a treeless plain, making it clear that the correct answer to the student's question is C. treeless plain. This is based on the information provided about the physical attributes of the tundra and its location in the northern hemisphere which is flat and typically has very little vegetation due to the cold and dry conditions.