The correct answer is C. The border between California and Oregon is a thin, black line; this indicates a state boundary.

Explanation

According to the map and its conventions, the correct option is C. because the line that divides the states of Oregon and California in the west of the country is a thin line that according to the conventions of the map indicates a “State Boundary”. Additionally, the other options are not correct, for example, option A. “The border between Texas and Mexico is a thin, black line; this indicates an international boundary” is incorrect because according to the conventions of the map, the “National Boundary” is indicated by two thick and continuous lines; in addition, option B. “The border between California and Oregon is a blue line; this indicates an international boundary” is incorrect because the state limit between Oregon and California is not a State River Boundary; finally, option D. “The border between Texas and Mexico is a thick, black line; this indicates a state boundary” is incorrect because the boundary between Mexico and Texas is an national boundary. So, the correct aswer is C. The border between California and Oregon is a thin, black line; this indicates a state boundary.