Among the options provided, a socialist oligarchy is the closest to a government that can have unlimited power. In a socialist oligarchy, a small group of individuals or a single party controls the political power without significant checks and balances, which can lead to a concentration of power and limited political freedoms.
In contrast:
- Unitary presidency can have significant power, but it typically operates within a constitutional framework that may impose limits.
- Parliamentary system involves multiple parties and a parliament that holds power, thus does not have unlimited authority.
- Constitutional monarchy operates under a constitution that limits the powers of the monarchy and often includes a parliamentary system, again preventing unlimited power.
Thus, the most appropriate answer is socialist oligarchy.