Final answer:
The smallest type of internal boundary in the United States is a precinct, which is used for electoral purposes. The closest option provided is 'intracity', which may refer to divisions within a city, such as precincts.
Step-by-step explanation:
In the United States, the smallest type of internal boundary is a precinct. A precinct is essentially a neighborhood-sized area that is used for electoral purposes, such as voting. It is smaller than a city, county, or state, and in urban areas, it typically consists of just a few city blocks. Precincts are part of multiple, larger electoral districts including wards, townships, counties, and states.
The correct answer to the student's question is therefore not expressly listed among the options provided. However, based on the context given on electoral districts and the smallest region one can vote in, the closest answer and implication would be (c) intracity, assuming intracity refers to divisions within a city, such as precincts.
Therefore, the answer is c. intracity. Please note that the student should verify this with the context of the choices provided in their material, as the term 'precinct' is not listed explicitly in the options.