Why wouldn’t Montana be considered "complete within" the United States?

Montana is not considered "complete within" the United States primarily because it shares an international border with Canada. The concept of being "complete within" refers to a geographic area that has defined boundaries that do not extend beyond a country's borders. In Montana's case, the international boundary signifies that part of its borders goes beyond the confines of the United States, making it an exception in this context.

A state that is entirely contained within the U.S. would not have any external borders linking it to other countries. Therefore, the presence of an international border means that Montana cannot be classified as fully enclosed or "complete within" the U.S., as part of its geographic detail extends into another nation.

The other options do not accurately address the concept of being "complete within." For instance, sharing a boundary with other states or being too large does not pertain to the fundamental geographic principle of national completeness, and having multiple entry points would not constitute a reason for Montana to be categorized differently in this context.