The correct answer is A. 100 microsecond.
Air blast circuit breakers are designed to interrupt high-current circuits. They do this by creating a blast of air that extinguishes the arc between the contacts. The speed at which the arc is extinguished is important, as it determines the amount of energy that is released in the system. A faster arc interruption results in less energy being released, which can reduce the risk of damage to the system.
Air blast circuit breakers for 400 kV power systems are designed to operate in 100 microseconds. This is because the higher the voltage, the faster the arc must be extinguished to prevent damage to the system.
Option B, 50 milliseconds, is too slow. This would result in too much energy being released in the system, which could cause damage.
Option C, 0.5 seconds, is also too slow. This would still result in too much energy being released in the system, and it would also take too long to interrupt the circuit, which could cause problems for the system.
Option D, 0.1 seconds, is the fastest of the options, but it is still too slow for a 400 kV power system. The arc must be extinguished in 100 microseconds or less to prevent damage to the system.