Efficient telephonic communications in any below ground coal mine should be provided when a system of haulage roadways & conveyors extends to a distance of more than 'A' meters. 'A' is given by

According to the provisions relating to communication systems in underground coal mines, efficient telephonic communication must be provided when a system of haulage roadways or conveyors extends to a distance of more than 300 meters from the shaft, incline, or other main entrance to the mine.

The purpose of this requirement is to ensure quick transmission of information regarding operational activities, emergencies, accidents, equipment failures, and safety instructions.

In underground mining operations, communication plays a vital role in maintaining worker safety, coordination of transport systems, and emergency response. As the distance increases, direct verbal communication becomes difficult and unreliable.

Therefore, mine safety regulations mandate the installation of an effective telephonic communication system once the haulage roadways or conveyor network extends beyond the specified limit.

Among the given options, the prescribed distance is 300 meters.

Hence, Option D (300 m) is the correct answer.