Answer:
A political unit within a state.
Step-by-step explanation:
A nation is a political organization with a defined geographic region, which may have one or more states within and it is a legitimate authority with international diplomatic recognition (so this option is wrong An imagined political community).
In the territory of nation people may share some common traits such as language and culture, but it may have more than one ethnic group, like Africa that has many ethnic groups (so this option is wrong A territory with one ethnic group).
An autonomous country has a self-Regulating government, but not all nations are autonomous, like Somaliland (so this option is wrong An autonomous country).
But many states might form a nation as a group and the nation will have a Constitution representing all the states and their interests (then the option correct is A political unit within a state).