Concept:Inter-departmental communication means the exchange of messages or information between different departments inside the same organisation.
Explanation:An intercom is a voice-based system that connects different rooms, offices, or departments within the same building.
It allows quick and direct spoken communication without moving from one department to another.
Notice boards, radiophones, and circulars are usually used for broader or one-way communication, not specifically for direct inter-departmental exchange.
Therefore, the most suitable medium for inter-departmental communication is the intercom.
Answer:A. intercom