**Classification of Power System Stability**

Power system stability refers to the ability of a power system to maintain a steady, synchronized operation under normal and abnormal conditions. It is crucial for the reliable and efficient operation of the electrical grid. Power system stability can be classified into three main categories:

**1. Frequency Stability:**
Frequency stability refers to the ability of a power system to maintain a stable frequency during normal and abnormal operating conditions. The frequency of an electrical system is determined by the balance between the generation and consumption of electrical power. Any imbalance between generation and load can cause frequency deviations.

**2. Voltage Stability:**
Voltage stability refers to the ability of a power system to maintain a stable voltage profile under normal and abnormal conditions. Voltage fluctuations can lead to equipment malfunctions, damage to electrical devices, and disruption of power supply. Voltage stability is influenced by factors such as load variations, reactive power demand, and system configuration.

**3. Rotor Angle Stability:**
Rotor angle stability, also known as transient stability, refers to the ability of a power system to maintain synchronism and restore stability after a disturbance. It specifically focuses on the stability of the synchronous machines in the power system. Rotor angle stability is critical for maintaining the overall stability of the power system and preventing cascading failures.

**Why the correct answer is option 'D':**
The correct answer is option 'D' - All of these because frequency stability, voltage stability, and rotor angle stability are all different aspects of power system stability. Each aspect addresses a specific characteristic and stability concern of the power system. These three classifications are interrelated and collectively contribute to the overall stability and reliability of the power system.

Frequency stability ensures that the system frequency remains within acceptable limits, preventing excessive speed changes in synchronous generators. Voltage stability ensures that the system voltage remains within acceptable limits, preventing voltage collapse and blackouts. Rotor angle stability ensures that the synchronous generators maintain synchronism and remain stable after disturbances, preventing cascading failures.

Therefore, all three classifications of power system stability - frequency stability, voltage stability, and rotor angle stability - are essential for the reliable operation of the electrical grid and the prevention of power system failures.