Among the options provided, a "socialist oligarchy" is the government that is often associated with having unlimited power. In such a system, a small group of individuals holds significant power and control, often without effective checks and balances.
The other options typically have structures in place that distribute power or impose limits on authority.
- Unitary presidency can have significant power, but it is usually subject to laws and a constitution.
- Parliamentary system features a government accountable to parliament and therefore has checks on power.
- Constitutional monarchy combines a monarch with a constitution, placing limits on the monarch's powers.
Thus, "socialist oligarchy" is the most fitting answer.