The methods employed in improving the system stability are

A. Increasing the system voltage only

B. Reducing the transfer reactance only 

C. Using high-speed auto-reclosing circuit breaker only

D. Using all (a), (b), (c) together 

Answer: D. Using all (a), (b), (c) together 

Explanation: 

Transient stability: Transient stability is the ability of the power system to maintain synchronism when subjected to a severe transient disturbance.

Transient stability of the system can be improved by

• Increasing the system voltage
• Increase in the X/R ratio
• ​Using high-speed governors on machines
• High-speed circuit breakers help to clear the fault as quickly as possible
• By turbine fast valving
• High-speed excitation
• Use of auto re-closing breakers
• Some of the other ways to improve the transient stability are by employing lightning arresters, high neutral grounding impedance, single pole switching, quick Automatic Voltage Regulators (AVRs).