Option C (Plain) is correct because the textbook clearly distinguishes between physical features and social/cultural features. The context states that the earth has variations in its physical features such as 'mountains, hills, valleys, plains, plateaus, oceans, lakes, deserts and wilderness.' On the other hand, 'roads, railways, ports, markets and many other elements' are listed as social and cultural features created by human beings. Therefore, a plain is a physical feature, while port, road, and water park are human-made features.