Analysis of Given Scenario:
The scenario describes a slack bus in a power system where certain parameters are known and unknown. Let's breakdown each parameter:

Known and Unknown Parameters:
- δ (delta) is unknown: The phase angle at the slack bus is unknown, which is a common characteristic of a slack bus.
- P is known: The real power injected at the slack bus is known, as it is typically a specified value in power system analysis.
- Q is unknown: The reactive power injected at the slack bus is unknown. This is not uncommon as the system may regulate reactive power flow through other buses.
- V is unknown: The voltage magnitude at the slack bus is also unknown. The voltage may be controlled and adjusted based on system requirements.

Explanation of Correct Answer:
The correct answer is option 'C' - Q is unknown. This is because while the real power P injected at the slack bus is usually a specified value, the reactive power Q may vary based on system conditions and requirements. Therefore, the reactive power Q at the slack bus is typically an unknown parameter in power system analysis.
In conclusion, the unknown parameters at a slack bus include the phase angle δ, reactive power Q, and voltage magnitude V. These parameters may need to be determined through power flow analysis and system studies to ensure proper operation and control of the power system.