Concept: Traditional communication includes both audible instruments and visual symbols, and a palm frond is a visual symbol used to indicate festivals.
Explanation: A palm frond is a symbolic medium of communication in African traditional society. It is often placed at gateways or on buildings to announce that a festival, ceremony, or special celebration is taking place.
Talking drums and metal gongs are mainly used to pass verbal messages or summon people over distances, not specifically to mark festivals.
Gum blast is not a recognized traditional medium of communication.
Therefore, among the options, palm frond best fits the role of marking festivals.
Answer: A. Palm frond