Explanation:

Introduction:
An urgent message can be sent through various means depending on the level of urgency and the available communication infrastructure. In this case, the correct option is 'B', which is Telegraph.

Telegraph:
- Telegraph is a communication system that uses electrical signals to transmit messages over long distances.
- It was widely used for urgent communication in the past, especially before the advent of modern technologies like the internet and mobile phones.
- Telegraph messages can be sent and received through telegraph offices or stations, where trained operators transmit and decode the messages.
- The messages are transmitted using Morse code, which is a series of dots and dashes representing letters and numbers.
- Telegraph messages can be sent quickly, making it suitable for urgent communication.

Postal envelope:
- Postal envelopes are used for sending physical mail or documents through postal services. They are not specifically designed for urgent communication.
- Postal services usually have a standard delivery time, which may not be suitable for urgent messages.
- While postal envelopes can be used to send important or time-sensitive documents, the urgency of the message may not be guaranteed.

Pigeon:
- Pigeons have been historically used as messengers due to their ability to fly long distances and return to their home location.
- However, sending urgent messages through pigeons is not a reliable or efficient method, especially in modern times.
- Pigeons are slower compared to other forms of communication and can be affected by weather conditions or other external factors.
- Additionally, relying on pigeons for urgent communication may not be feasible in areas without a trained pigeon network.

Satellite:
- Satellites are used for various communication purposes, including transmitting urgent messages.
- However, satellite communication typically requires specialized equipment and infrastructure, making it less accessible for urgent communication compared to telegraph.
- While satellites can transmit messages quickly over long distances, they may not be the most practical option for urgent communication in all situations.

Conclusion:
In summary, while there are various means of communication available, the most suitable option for sending an urgent message is telegraph. Telegraph allows for quick transmission of messages over long distances, making it a reliable method for urgent communication.