In the united states, a(n) boundary separates us lands from another country.
a.
county
b.
intracity
c.
federal
d.
state

C, the reason this is the answer is because federal usually refers to the government, and the border between two countries is always government owned.
An example of this is the border between Mexico and the United States.

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C.  federal

Explanation:

The U.S has about 50 of the states as they widely distributed in terms of the languages and their boundaries are a modified type by the federal government during the 1780s and the '90s these were set by the congress as it created territories. Often following the various geographical features such as a river a mountain and peaks, and were influenced by the transportation network of the route and thus divided as accordingly.