Final answer:
The best explanation for the shape of the solid line between point A and D on a map is a political boundary, often based on natural geographic features such as rivers and mountain ranges.
Step-by-step explanation:
The solid line between point A and D on a map could best be accounted for by a political boundary. Political boundaries are often influenced by and coincide with natural physical geographic features such as rivers, mountain ranges, and coastlines. For instance, the Rio Grande serves as a portion of the border between the United States and Mexico, while the Great Lakes and Saint Lawrence Seaway form part of the border between Canada and the United States. Additionally, mountain ranges like the Alps and the Himalayas act as natural boundaries between countries. These features not only define regions but can also be the cause of disputes when resources are involved, further emphasizing their role in shaping political territories.